IN one form, it is as creamy and delicate as the finest silk. In another form, it has the luxurious, nappy character of high-quality suede. And without much popular notice, it has found its way into the collections of the trendiest designers in the world.What is this new fabric of the '90s? Polyester, thank you. That disparaged step-child of high fashion may be on its way to sartorial greatness - thanks to the development of "microfiber," a high-tech version of polyester that's making waves in the international fashion world for its "chameleon" quality and durability. If there ever could be a "miracle" fiber, microfiber fits the bill, designers and fashion experts say. Four to 15 times thinner than the average human hair, microfiber filaments can be used to create fabric that imitates the look and feel of high-quality wool, silk, chiffon, linen, suede - even leather. Manufacturers of microfiber are going on record saying that it's washable, wrinkle free, water repellent, and breathable. Top designers such as Donna Karan, Bill Blass, Calvin Klein, Liz Claiborne, Giorgio Armani, Peter Hadley, and Max Mara are producing pure microfiber garments or blending it with natural fibers. Though its application extends to ties, suits, dresses, and blouses, its most obvious use has been in raincoats and jackets that stand out for their plush "peach skin" texture. "I don't think it's a fad at all. It's a very functional, long-term kind of fabric," says designer Nicole Miller, who first offered microfiber raincoats last spring. "They did so well that I ran them in new colors for fall," she says. Bloomingdale's, Macy's, Charivari, and her own store on Madison Avenue are among stores that are carrying them. Microfiber, highly valued in Europe and catching on in America, is part of a general fashion trend toward garments that are stylish yet practical. Polyester is virtually indestructible, while natural fibers like cotton, wool, and silk present ironing and cleaning hassles. "As we've come into the '90s ... people want ease of care with clothing," says Caryl Svendsen-Deiches, spokeswoman for the Sewing Fashion Council in New York, which represents many American fiber companies and the home sewing industry. "They don't want to go to the laundry cleaners every week." Microfiber manufacturers are not unaware, however, of Americans' aversion to polyester clothing. The petroleum-based synthetic first burst on scene in the 1960s - only to be booed off the high-fashion runways by designers and consumers who considered it too cheap, ill-fitting, and sweaty for anyone with a microbe of good taste. But many American designers, impressed with microfiber's versatility and luxurious quality, are now signaling thumbs up. A prohibitive price tag in the designer category has added to its cachet. Last fall, top-of-the-line Italian microfiber was selling for as much as $50 a yard wholesale. Some American and Japanese fabric companies, however, are producing less expensive versions. Toray Industries Inc., a Japanese company, offers a microfiber crepe for $6 a yard. In America, microfiber is being carefully marketed. One microfiber trench coat, made by Searle, sells for $698 at a Boston Saks Fifth Avenue store. To play down the polyester content, a dangling tag reads "Shamois," a "unique man-made fabric." "No one's talking polyester," says Frank Rizzo, chairman of the fashion department at the Parsons School of Design. "In Italian, it even sounds chic. It's 'mee-cro-FEE-breh. Fabric companies are marketing it with swanky names: Micromattique, Shingosen, Ultra Fiber, Microma, MicroSpun, Trevira Finesse, and Tactel Micro. Microfiber, originally invented in Japan a number of years ago, is defined as fabric that uses "microdenier" polyester yarn - four times finer than cotton fibers, and finer than the finest natural silk. An Italian company, Montefibre, has made a microfilament yarn called Terital Zero.4 that's so thin it would take less than five pounds of it to circle the earth around the equator. The high-density, tight lattice of microfiber accounts for its durability - as well as its soft touch. And because it's strong, it can be brushed or finished to produce a variety of rich textures and appearances. "What they [the fabric companies] are doing with it is brilliant," says Rizzo, who visits international fabric shows each year. "They can do a sueded silk that's better than all the washed silks you've ever seen. It washes beautifully without losing color, it drip dries, and it's like butter - you can roll it up in your hands. The old polyesters were rigid ... and had a tendency to be puffy and kind of stick out from the body. The new microfibers are like fluid on the body." Because microfilaments are so thin, when they are bundled together to make yarn, the yarn has more air "pockets" allowing perspiration to escape. In some types of microfiber, the spaces are too small for water droplets to get in, making garments light and comfortable - but waterproof. "The fabric breathes very well, and it's similar in this sense to the natural fibers," says Francesca Colloredo, spokeswoman for the Italian Silk Association in Milan, reached by phone. "That's why it's been very successful, especially in sportswear. It's big news in our industry," she adds, "because the texture is very silky, so it blends very well with the other silks we use." Blending microfiber with wool, for instance, gives the wool characteristics it didn't have before. "Wool is not very strong in itself, but when you blend it with a microfiber polyester, ... you can launder it,... and you can also get colors you couldn't before," says Ms. Svendsen-Deiches. Whether microfiber will become as popular in America as it is in Europe is unclear, although an impressive number of designers, fashion houses, and textile companies are vigorously promoting it. "I think microfiber will get stronger and stronger," says Thanos Kamiliotis, president of E. Boselli & Co., an Italian polyester fabric company. Calvin Klein, Helen Tracy, Sanyo (maker of raincoats), and Searle have recently purchased microfiber from him. "Any designer who wants to do business, they have to comply with consumer demands," he says. In America, "I think microfiber is the savior [of polyester]." In the lower price-point category, the national womenswear chain called Units introduced microfiber this past spring into their line of mixable separates. "It did very well," says Cari Nolan, public relations manager for the Dallas-based company. "We've included it again in our fall and holiday collections." Ms. Nolan says Units has received an "incredible response" to a new MicroSpun pleated skirt and blouse that look like silk, but can be washed and tumble dried. The price is $128.