A woman running naked through the jungle on Blood Island is killed by a green-skinned beast that resembles a man. At the same time, a ship arrives at the island carrying American pathologist Bill Foster, who is investigating a strange chlorophyll disease among the islanders; Sheila Willard, who has come to Blood Island seeking to reunite with her father; and Carlos Lopez (Ronaldo Valdez), who wants to get his mother, Mrs. Lopez, to move off the island. The captain of the ship claims that the island is cursed and tells a story of a man they picked up on a raft who bled green blood before he escaped into the sea.

Sheila discovers that her father is a hopeless alcoholic. Mrs. Lopez does not want to leave the island, even though her husband, Don Ramon Lopez, died there recently under mysterious circumstances. The suspicious Dr. Lorca will not reveal any details about Don Ramon's death to anyone. When Foster and Lopez exhume the grave of Don Ramon, it is empty.

Rumors abound about a green-skinned monster with chlorophyll blood that has been killing the local natives. One night, a native with green sores on his body tries to break into the government house, but runs off into the jungle when he is confronted. Sheila is attacked in the jungle by the chlorophyll monster, but manages to escape when an unfortunate native who comes to her rescue is gruesomely mutilated by the creature. Sheila and Dr. Foster fall in love during their stay on the island.

It is revealed that Dr. Lorca has been experimenting on the natives, including the unfortunate Don Ramon, who had sought Dr. Lorca's serum as a treatment for his cancer, but was turned into a monster instead. They learn that Don Ramon is actually the green-blooded beast that has been killing people on the island. Don Ramon kills his wife, and almost kills his son Carlos, but at the last moment a glimmer of humanity returns to the creature and realizing what a monster he has become, he attacks Dr. Lorca in his hidden lab instead. A fire breaks out in Lorca's lab, resulting in a huge explosion, killing Dr. Lorca, his assistant and the monster.

Sheila, her father, Dr. Foster and Carlos all return to the ship, glad to finally leave Blood Island. But as the ship leaves port, a grisly hand appears from underneath a boat tarp, dripping green blood.

• John Ashley as Dr. Bill Foster
• Angelique Pettyjohn as Sheila Willard
• Ronald Remy as Dr. Lorca
• Alicia Alonzo as Marla
• Ronaldo Valdez as Carlos Lopez
• Tita Muñoz as Mrs. Lopez
• Tony Edmunds as Mr. Willard
• Alfonso Carvajal as Ramu
• Bruno Punzalan as Razak
• Edward D. Murphy as Captain
• Eddie Garcia as the monster

Prior to production on The Mad Doctor of Blood Island, Ashley had starred in Brides of Blood. The film was popular enough in the United States to get American distributors to ask him to appear in a follow-up film.^[1]: 77 Ashley agreed, which led to his moving to the Philippines and co-producing several other films there in partnership with Romero, Beverly Miller and Roger Corman.^{[1]: 77 [3]}

Mad Doctor was produced by Romero and Kane W. Lynn along with Miller and Irwin Pizor,^[2][1]: 74 on a budget of either $125,000 or $100,000.^[4][5]

The score was composed by Tito Arevalo.^[1]: 83

A prologue to the film invited theatergoers to partake in a bizarre initiation, and each patron was given a free packet of colored liquid labelled "green blood". At a certain point, the audience was told to "recite the oath of green blood" as they tore open the little packets and drank the colored liquid inside. By doing this, the viewer could safely watch "the unnatural green-blooded ones without fear of contamination".^[2] The prologue was shot at Clark Air Base in Manila using American teenagers.^{[1]: 63–88}

Sam Sherman came up with the idea of distributing the liquid gel-packs to the theatergoers, and said years later in an interview that he drank one of the packets, which contained an aqua-colored gel, and it made him sick to his stomach. Nevertheless, Miller (an associate producer of the film) said he actually witnessed dozens of teenagers drinking the stuff in the theaters that he managed in Kansas City.^[1]: 74

In another gimmick, the camera zoomed in and out rapidly each time the monster killed or stalked someone, a technique that some theatergoers complained made them dizzy and was actually designed to cover up the shoddy make-up effects.^[2][1]: 72

The film was released in the U.S. in 1969 on a double feature with the 1967 West German film The Blood Demon.^{[1]: 74 [1]: 83} In 1969, a practice arose in some states (Rhode Island in particular) wherein the local newspapers began omitting the word "blood" from the titles of films they were advertising. In certain areas, Mad Doctor was advertised as The Mad Doctor of Crimson Island and The Blood Demon became The Crimson Demon, etc. This practice did not last very long, and by the time the sequel Beast of Blood was released, Hemisphere was again able to use the word "blood" again in their titles.^[1]: 74

Image Entertainment released Mad Doctor on DVD in 2002, featuring a commentary track by Sherman and an interview with Romero.^[6]

Mad Doctor of Blood Island received mostly negative reviews from critics.

Dennis Schwartz from Ozus' World Movie Reviews awarded the film a C grade, calling it "dreary" and stating that the film "is as bad as it sounds".^[7] On his website Fantastic Movie Musings and Ramblings, Dave Sindelar called it " intermittently fun but intermittently disappointing as well".^[8] Paul Gaita from Allmovie liked the film. While noting the film's poor camerawork, Gaita stated that the film "has a loopy charm that will be best appreciated by fans of low-budget horror".^[9] TV Guide gave the film 1 out of 5 stars, criticizing the film's dialogue and overuse of zoom shots.^[10]

The film proved to be popular commercially, and was followed by the last film in the series, Beast of Blood (1970), which saw the return of the chlorophyll creature.

Romero later said of Mad Doctor: "We thought it was one of the worst things we ever did... I can't account for it [the film's impact]".^[11]

• Terror Is a Man
• Brides of Blood
• Beast of Blood (film)

• The Mad Doctor of Blood Island at AllMovie
• The Mad Doctor of Blood Island at IMDb  
• The Mad Doctor of Blood Island at Rotten Tomatoes
• The Mad Doctor of Blood Island at the TCM Movie Database