LOS ANGELES, June 12 — Hollywood has been circulating movie trailers on the Web for years, but only now is the film industry retrofitting its rating system to give the studios a chance to showcase their racier material online.

No matter what the rating of the film, nearly all the trailers shown in theaters — and on the Web — have come with a so-called green band, or tag, saying they are approved for all audiences by the Motion Picture Association of America. For movies rated PG-13 or stronger, that often meant watering down the violence, sex, language and overall intensity of a trailer.

But in April a teaser trailer for Rob Zombie’s “Halloween” remake, set for release on Aug. 31, became the first to display a new yellow tag signaling that it was “approved only for age-appropriate Internet users” — defined by the Motion Picture Association as visitors to sites either frequented mainly by grown-ups or accessible only between 9 p.m. and 4 a.m.

And two raunchy comedies — “Knocked Up” and this August’s “Superbad” — are among a spate of recent films with R-rated, “red-tag” Internet trailers, which require viewers to pass an age-verification test, in which viewers 17 and older have to match their names, birthdays and ZIP codes against public records on file.