Pierre Cardin may have revolutioned the Haute-couture industry yesterday by introducing to the international world of expensive fashion something American schoolchildren have known for years; The Hefty trash bag look.

The garment, which doubles as beach jacket and raincoat, looks remarkably akin to a five-gallon-size green plastic garbage bag with slits cut in the top and sides for heads and arms.

Of course, the noted French designer has taken it one step further. The jacket is lashed to the body with generous circuits of green plastic lacing several inches wide, giving the person enconced in it the look of an October Saturday's raked leaves ready to be put out for the sanitation engineers.

This could have a devastating effect on fashion houses. No longer will women need to spend the hundreds of dollars that these items normally cost. Nor will they have to wait while Cardin's workers make the jacket to order. A quick trip to the supermarket for plastic bags and a carefully wielded scissors will have even the most harried woman right on the haute-couture path.

Not all of Cardin's introductions - one of eight collections of spring/summer haute-couture fashions shown here yesterday - will be as easy to copy. There was, for example, his inverted cowboy hat, which has the crown of the standard 10-gallon items removed so that the brim radiates out from a bare head; a dress reminiscent of those worn by Popeye's true-love Olive Oyl; a slinky tube with pockets identical to its short sleeves, both set at 90-degree angles to the main cylinder in the style of stove piping; and the perfect evening-wear for 1978's Bonnie Parker; Formal dress capped by a contoured veil that covers the entire head, quite akin to the common nylon stocking worn by the most bank robbers.

The first of four days of fashion introductions began - somewhat awkwardly - yesterday at 9 a.m. with the extremely balloony pants and dresses of Jacques Esterel. As the first of his models stepped out from a stage designed to look like a beach, she tripped and nearly came crashing down along with the scenery. The lights went black, the pulsating disco music halted, and two minutes later it all started again. Looking like the Red Queen in "Through the Looking Glass," Esterel's model finally managed to dance down the runway - no mean feat at 9 in the morning; and dressed in an outfit that most mortal beings would trip over.

All this was climaxed - as the Beatles' "When I'm 64" came through the sound system - by Esterel's billowing, '50s-ish bridal gown, best described as your basic maternity wedding dress. Somewhere underneath all those viels and layers of chiffon a bride could easily hide anything - not to mention twins or even quintuplets.

The day's major applause - from the horde of gawkers, journalists and would-be buyers gathered here for the twice-annual fashion romp - came for the designs of Jean-Louis Scherrer, who has taken the "Annie Hall" women-wearing-ties-look one step further. As the designer says in his own press release:

"Young English lords . . . Charlie Chaplin's heroines . . . an elegance found in old English engravings . . . a collection that suggests a certain era, the image of a memory that remains in today's spirit, a certain softness, humor, fragile women . . . squared shoulders, short jackets, thin neckties underwinged collars, French cuffs . . . a small derby hat made feminine by curls peeking out . . . the time of an image, the reflection of a dream, the necessity of being romantic in 'modern times' . . ."

In fact, Scherrer's models do have a romantic air, albeit an unusual one. They never smile, and seem to have just slipped from some pond of Botticelli, bound right for a rendevous where those dainty little men's outfits are going to be in for a rough time.

In contrast to this, Christian Dior - yesterday's other major show - opted for a look of "classicism and suppleness, whimsy and charm." It was easily the most packed house of the day although the crowd responded rather tepidly.

Dior's constantly smiling models seemed right at home in the ornately gilded mansion that houses the company, with their endless cascades of gabardine and silk. The multilayered outfits were so complex that assistants were stationed regularly through the crowd to help the models put on pieces they had taken off.

That may be the price of whimsy or charm.

Or, as one model said to another, after she thought she was out of the crowd's ear-range:

"Get me out of this thing and into a pair of jeans."