Menswear designers successfully venturing into women's fashions

In an interesting switch on an established pattern, menswear designers like Alexander Julian, Henry Grethel, and Ron Chereskin, having made their mark in male apparel in a few short years, are venturing confidently into women's fashions.

It's a turnaround from the former routine set by designers like Bill Blass, Oscar de la Renta, Calvin Klein, Yves Saint Laurent, and Donald Brooks. They went the glamour route for many years, became very successful, then, as though looking for a fresh challenge, got into menswear design.

Although the first women's garments by Julian, Grethel, Chereskin, and others reflected the tailored shapes, colors, and fabrics of male attire, not so the new spring and summer offerings.

They're soft, versatile, colorful, and unmistakably feminine.

This is the second women's collection for Ron Chereskin. The first one premiered last fall. It focuses on separates dressing, with a few surprises such as a simple linen chemise and a jump suit.

In between crisp black linen jackets and stark white cotton sundresses are pale and bright colors like soft pink, tomato red, and turquoise; also subtle stripes and bold brushstroke prints.

''I think warm weather is a time for easy, unconstructed clothes, no gimmicks or frills, just straightforward clothes that work for many occasions,'' Mr. Chereskin says.

Henry Grethel debuted his first women's line in 1980. It won early acceptance and the popularity seems to be continuing.

It has also spread to other countries, first Britain and Australia, now in 1984 to Canada and Japan.

''Design has become international today,'' says Mr. Grethel, who in 1982 won the Dallas Fashion Award for his women's sportswear. ''You can be in New York, Tokyo, or Paris and see an astonishingly similar design aesthetic.''

For spring and summer he is introducing romantic Milanese linens and a fresh-looking nautical chambray group. Pleated skirts and pajama-style walk shorts are other newsmakers, while two-piece dressing raises a flag with regimental stripes.

Mr. Grethel has designed a large shirt collection with an eye to femininity.

''I believe shirts offer a woman accessory options and (less expensive) ways to expand her wardrobe. For spring I designed soft-patterned shirts in fluid, oversize shapes to underscore femininity.''

The collection also makes a point of shirt dresses, big comfortable jump suits, fuller pants, and knitted jersey outfits.

To sum up the feeling of his third women's line, master colorist Alexander Julian says, ''I'm concentrating on fabrics that breathe, easy construction, and simple shapes.''

There's an emphasis throughout on Italian linens in unusual stripes and weaves and, as may be expected, an exciting interplay of colors.

Angular seaming helps to create Mr. Julian's long, lean chemise silhouettes and tunics. Focus also is on dropped waists, sashed pants, apron skirts, and hip-grazing jackets.

Perhaps most logical of all is Jhane Barnes's entry into female design after the impressive success of her innovative men's fashions. To her women's line she brings the marvelous fabrics she creates herself, adapting them to soft but sophisticated shapes, like one summer suit with deep dolman sleeves.

For evening, the award-winning designer does a dramatic cocoon dress of silk broadcloth with a deep V-neck in back framed by a cowl.