Some people can't be trusted with a gimmick that can't miss. For a textbook example, consider the schlock movie anthologists responsible for "It Came From Hollywood." They've contrived to make a mockery of their own idea for a humorous novelty film by gratuitously mocking the source material -- delightfully absurd clips from dozens of harmlessly atrocious low-budget horror thrillers, science-fiction spectacles and teen-age exploitation melodramas. These were usually improvised into hasty, defective, irresistibly silly existence in the B to Z production enclaves of Hollywood during the '50s and early '60s.

If it worked properly, "It Came From Hollywood" might have endeared itself to movie nuts as a "That's Entertainment!" of derelict films, or a kind of flip-book big-screen digest of "The 50 Worst Films of All Time," whose authors, Charles and Harry Medved, are listed as consultants in the depths of the end credits. It's not as if the codirectors, Malcolm Leo and Andrew Solt, lacked experience with the format. Between them they've been involved with dozens of TV specials and show-business compilation documentaries, including "Life Goes to the Movies," "Heroes of Rock 'n' Roll," "Bob Hope's Overseas Christmas Tours" and "This Is Elvis."

What they appear to lack on this occasion -- and I'm convinced it's a deficiency that will cost them severely at the box office -- is confidence in the enduring entertainment value of the treasure trove of ridiculousness at their disposal. This anxiety has prompted them to get desperately facetious with material that is inherently funny. A group of comedians presumed to be reliable attractions for a youngish movie public -- Dan Aykroyd, John Candy, Gilda Radner and Cheech & Chong -- has been recruited not only to introduce the old clips, which wouldn't be such a bad idea, but also to upstage them with wisecracks and funny business of their own, which proves a wrongheaded, self-defeating idea.

To his credit, Candy seems to resist the general tendency to act superior to the source material. He's drawn into a dumb skit with Aykroyd after narrating a segment devoted to highlights from the inimitable oeuvre of the late Edward D. Wood Jr., generally considered the king of awful directors -- several choice moments from "Plan Nine from Outer Space" and a priceless climactic confrontation from "Glen or Glenda," Wood's autobiographical meditation on transvestism. But as a rule Candy refrains from scoring easy points off the clips. He also tends to mention the titles, a little amenity that is seldom observed by the other narrators.

Aykroyd, Radner and Cheech & Chong should never have been encouraged to compete with these vintage excerpts by inserting their own shtick and sarcastic comments, which are usually nothing to get smug about. Aykroyd and Radner become so frenetic that you'd think they were still unknowns, anxiously overdoing it for a big audition. But quite apart from the poor quality of their clowning and wisecracking, they should really have enough regard for other performers, including the obscure and hopeless amateurs, to avoid heaping ridicule on their embarrassing moments.

For example, it's offensive when Cheech Marin mimics some unknown actor pretending to go berserk in an unidentified horror movie. Marin is also guilty of the single stupidest ad lib, inspired by a shot of a panicky addict getting rid of his reefer -- "Hey, man, there's a lotta kids starving in Cambodia who want that roach."

At 80 minutes, "It Came From Hollywood" scarcely justifies itself as a theatrical attraction. The narrators keep shuttling in and out to joke about the footage in segments that seem arbitrarily organized (Gilda Radner Presents Gorillas, Dan Aykroyd Presents Aliens, etc., etc.) and then intensify the annoyance by shortchanging the nominal topics. There are occasional smart juxtapositions, like the wonderfully complementary jump from "The Amazing Colossal Man" to "The Attack of the Fifty Foot Woman" and the clever use of a straight line from "I Married a Monster from Outer Space" to cue the trailer for "Space Children." When the compilers allow excerpts to unfold in their own goofy way, without interference or nudging, you're reminded that much of the inadvertent comic impact and charm of terrible movies derives from their sincerity.

"It Came From Hollywood" violates a basic trust with its admittedly shabby source material -- and with the human comedy, when you get right down to it -- by failing to show a proper respect for movies that dare to be utterly ridiculous and sometimes surpass themselves by becoming sublimely ridiculous.