This must have been one of those drive-in movies that nobody ever actually watched. It falls into the category of other 1950's horror movies that lack recognizable talent, an interesting script, and anything even remotely suspenseful or scary, unless you think that a hunchback is scary. They did manage to write a gorilla into the film, so I give them some credit for originality, but not much.
The effects are pretty bad too. There is one scene where one of the bad guys fires a gun. We can only tell it's been fired because we hear a gunshot and the target crumples bloodlessly to the floor. The gorilla is obviously a man in a suit, although I suppose the budget to actually get a real gorilla was probably way beyond this film.
The story is about four teens that go boating out to an island, where they are captured by a scientist working on a drug to make the American populace mindless and obedient. The term "zombie" is hardly ever used in the film at all, and there is only one "zombie" that we even see close up for most of the film.
Two more friends start a search when the other four fail to meet them, and the story progresses from there. I was mildly interested during the fight scene, but by then so much of the movie had gone by without drawing me in that I couldn't bring myself to care too much.