Crisp linen. Breezy seersucker. Long-wearing cotton. Cool comfort is what designers are striving for in their '86 sportswear collections.

THE mood for outdoor living this spring and summer is white, casual, and whimsical -- but not wild. Sportswear is being called ``classic sportswear,'' with linen, cotton, and sun-washed seersucker at the top of the list, in shorts, trousers, or easy dresses that breathe and flow in the summer breeze. Blue and white, black and white with a dotted or a zebra-stripe motif, and plain sparkling white are the designers' choices for the outdoors.

``It's a kind of sportswear that could last you a lifetime, because it's cool and wearable, and most of all, it's timeless,'' says Theresa Mantenfel, a Seventh Avenue fashion director. ``Good taste, simple lines, natural fabrics, and balanced colors inevitably win the day.''

Bathing suits, too, have sprung up in classic lines, with hints of the '20s and some surprising evening-wear touches, such as bows and ribbons. But there seem to be as many styles as there are tastes to wear them -- all the way from the risqu'e variety, which very few women would actually wear to a beach, to the more standard tank top, which crops up each season. -- H. N.