It was filmed in Japan in 1968, under the title "Battle Beyond the Stars" using a largely American cast of B-movie actors, including Robert Horton, Richard Jaeckel, and Luciana Paluzzi. It was co-written by a group of screenwriters active in B-movies at the time: Charles Sinclair, who wrote Batman (1966 film), Bill Finger, and Tom Rowe. Ivan Reiner, who co-wrote Wild, Wild Planet and other Italian-made science fiction movies of the 1960s, is credited with the story. Finger reputedly co-created the Batman comic book character and wrote or co-wrote other B-movies, such as Track of the Moon Beast. The director, Kinji Fukasaku, was a reliable and dynamic studio program director at the time, who was best known for his nihilistic gangster films (later unanimously lauded for The Yakuza Papers film series), maintains a frenetic pace for the narrative.
The American background players were recruited from the foreign community in Japan, as well as foreign amateur actors, which included Americans, Turks and Germans, many of who had extensive experience working for Japanese filmmakers. Background players were supplemented by USAF personnel from the Yokota Air Base near Tokyo, and female, American, fashion models based in Japan. Communication between the Japanese crew and the American actors was facilitated through ex-pat translators, namely production manager, William Ross, who founded his own English-dubbing facility in Japan, Frontier Enterprises.
The film's visual effects were provided by Japan Special Effects Co. and directed by Akira Watanabe, while the monster suits were created by Ekisu Productions; both companies formed by ex-Toho Studios employees, who worked under Eiji Tsuburaya, the acknowledged "Father of Japanese Special Effects." Children were recruited as "suit actors" to play the majority of the monsters.
The Japanese cut of Gamma 3: Operation Outer Space was 13 minutes shorter than the American version, and eliminated the Horton-Paluzzi-Jaeckel love triangle subplot (which would be boring for children), added different musical cues by Toshiaki Tsushima (notably absent was the infamous theme song), giving the film an increased action movie feel.
Green Slime's May 1969 U.S release was met with mostly negative critical reaction. The New York Times review was typical -- on May 22, 1969, reviewer Howard Thomson wrote, "The dialogue is wooden, so is most of the acting by a cast including Robert Horton, Richard Jaeckel and Luciana Paluzzi. And a dull and obvious romantic triangle continually squashes the terror potential at the ripest moments."
However, the movie did achieve a popular success with American matinée audiences due, in part, to an extensive advertising campaign aimed at children. The MPAA rated the film G.