The Flesh and Blood Show (1972)
More flesh than blood in this giallo-style British slasher. Starts with a couple of women in bed when somebody knocks on the door. The one who gets up to answer it is stark naked; you'd think her roommate, in baby doll nightwear, would have been the one to go see who's there. Anyway, there's a guy with a knife in his belly, bleeding profusely. Our first murder? No, it's just a joke.
The real plot starts when these folks, and some other actors, get invited to do some kind of show at an old abandoned theater by the sea. (This play-within-the-movie seems pretty bizarre, as it involves the actors dancing around in cavepeople costumes, then in leotards and such, and is said to have a "witchcraft scene.") They settle into the place for the night, offering more scenes of the women taking off their clothes, and our mandatory lesbian scene.
Eventually one of the women gets decapitated by the usual gloved killer, but her head and body are replaced by parts of a mannequin, so the cops dismiss the whole thing as a prank. More killings follow, in pretty slow fashion, until we get a long flashback sequence revealing the murderer's insane motive. You'll figure out whodunit long before this.
(This climactic sequence was filmed to be shown in 3-D, so it looks weird on the flat screen; black-and-white with red and green highlights. The flashback also offers full male as well as full female nudity, which is a refreshing change.)
Not a great film, but at least one older actor really gets to show off his stuff among all the younger performers.