The Helen (Leni) Stern collection of 100 works of contemporary art goes on sale today. The paintings, sculptures, drawings and prints constitute her share of a larger collection formed between 1958 and 1968, when Stern and her former husband, department store heir, author and philanthropist Philip Stern, player a major role in the contemporary art scene here.

Stern left Washington Sunday for Mexico, where she will devote her time, talent and money to helping the 1,200 children of Father William Wasson's orphan community, "Nuestros Pequenos Hermanos" (Our Little Brothers and Sisters), outside Cuernavaca.

The collection is being sold by the Pyramid Gallery to benefit that community. "If all goes well, we should raise a half million dollars - I hope," said Stern.

The most important works currently on view include two paintings by Kenneth Noland, a chevron and an early acrylic from the late '50s. The latter was purchased by Stern for $250 when Noland was driving a cab to make a living. Also of major interest are a stunning 1968 Sam Gilliam entitled "Cool Blue Shift," and Howard Mehring's "All Blue," currently loan at the National Collection of Fine Arts.

Other major painters represented are Marsden Hartley, Joseph Albers, John Marin, Vasarely and Jack Youngerman. Among numerous sculptors are Anthony Caro, Rockne Krebs, Ann Truitt and Reuben Nakian.

The single most valuable work in the collection, David Smith's "Cubi XXVI," was sold earlier this year to the National Gallery, and now stands at the entrance of the gallery's new Center for Advanced Studies. The rest of the collection will be shown on a rotating basis at Pyramid throughout the summer.

"This collection is studded with mementos, all of which have tremendous sentimental value to me," said Stern. "I'm keeping a few personal things for myself - works I can't part with - but the rest will have to be sold."

According to Pyramid, Stern's "list of things she can't part with" keeps getting longer, and now includes a Hans Hofmann gouache, a Frank Wright etching of "Don Quixote," some ceramics by Di and Lou Stovall, a Grandma Moses painting, a photograph by Charles Clapp, and several silkscreen prints by Jonathan Meader, "I can't let those go."

Stern, through the Stern Family Fund, helped several Washington artists like Gilliam, Krebs, Mehring and Truitt to get a toehold on the fast moving art world outside Washington by giving them time, space and materials when they needed them in the '60s. All have since made substantial international reputations.

Asked how she felt parting with her very personal collection, she replied, "I was thinking how I'd feel 15 years from now, if I saw my Marin on somebody else's wall. I'd have a pang. I know it. But I also know what I'm doing, and it's worth it."