The hottest fashion item of the moment at Sak's Fifth Avenue (next to Elsie's) is a smart gray and white striped seersucker dinner jacket. As tradition dictates, it has one button natural shoulders, flap pockets and shawl collar.

Fully lined, the dinner jacket sells for $45, goes everywhere a white dinner jacket can, and is suitable for all but the most formal affair. It is perfectly proper for late spring dinners and ideal for graduation week.

Madras will be much more important in men's wear this summer than last. Sports coats ($37.50) have already arrived at Sak's with labels reading: "Not colorfast: guaranteed to blood." Soon walking shorts, ties, shirts, and swim suits will also be available in madras.

Sak's dacron and wool gabardine suit is something to dream about in transitional wear. The traditionally styled three-button model ($85) is wrinkle resistant and perfect for travel as well as for hot sticky days. It comes in natural and dark olive.

J. Brine's clearance sale continues until Friday. Absolutely everything is reduced: both men's and women's sportswear, parkas, stretch pants, Italian hand-knit sweaters, skis and all equipment. Most items are 20 per cent off regular price. Mrs. Brine insists that her down-filled quilted parkas ($28) are "the warmest thing men can buy."

In women's wear, Brine's has a sizeable collection of conventionally styled wool sweaters, skirts and slacks from well-known manufacturers. Machine washable wool cardigan sweaters are irresistible at $6. Warm, lined boots (including U.S. Rubber's "Eskiloos") are still available in popular sizes. And last summer's sportswear is now on sale at half price.

Ski Pants and Bikinis

At the Ski Hut, on Charles Street, Boston, lovely Ingrid serves coffee in front of a cheery hearth and the sounds of Cal Tjader fill the room. Here you are treated as a guest as well as a customer who now can take advantage of a gigantic clearance sale on ski sweaters, stretch pants and equipment.

Although it specializes in ski wear and equipment, the store promises an extensive summer sportswear collection this year. There will be shifts of terry cloth, cotton, silk, and jersey including these of the Italian coutier Pucci based on wild Picasso silk screen prints. There will also be men's and women's sandals ($6 up) and bathing suits from Sweden and France.

For bikinis, the Ski Hut is definitely the place to go; already some bikinis as well as conventional suits, are in stock. One big hit is going to be the bikini in black vinyl ($17), fortunately sold with a short black vinyl coat ($21). Vinyl is a new plastic-like fabric which looks wet even without ever getting near the water.

The true bikini is forward upon at some Massachusetts beaches so the Ski Hut is also carrying well-cut, two piece stretch suits. These can be pulled up to the waist to beat the bikiat ban