ONE DAY last February Rudolf Serkin and his wife, Irene, braved the chills of winter to travel from their cozy Brattleboro, Vt., home to a concert that many would have considered obscure indeed.

"We very rarely go to New York and then we went only for that particular concert," recalled the pianist by phone last week. "It was one we did not want to miss."

What the Serkins did not want to miss was the New York debut of Washington's Theater Chamber Players, an increasingly prestigious instrumental group that specializes in juxtaposing rarely played works of the pre-19th Century period - that is to say, before the rich mainstream of the chamber music repertory - with complementary compositions of the present.

The Chamber Players seemed like a long shot indeed when the ensemble was first put together by Washington pianist Dina Koston and the piano virtuoso and conductor Leon Fleisher, who remain as co-directors.

But as the Players open their 11th season tomorrow night, at the Smithsonian's Baird Auditorium, it is clear that TCP was no pie-in-the-sky scheme.

One reason for it's success is the quality of the programming, which has brought a new dimension to music in this city. The Program tomorrow will be a typical mixture of the old (Parcell's Four Fantasias for strings), a contemporary masterpiece (the 2nd Bartok Quartet) and an American premiere "the Inscape for Soprano, narrotorm reeds and percussion by British composer Anthony Gilbert).

Another reason is the dogged and skillful fundraising, scheduling, recruitment and basic administration of Koston. "Sheis the backbone," said one member. "It has been a close call several times and she - at her own expense both in money and time - has uplled us through."

This season the years of work by the musicians will pay off when on Feb. 18 the group makes its debut as the resident company in the Kennedy Center's new Studio Theater. As such it will play a role here somewhat similar to New York's Chamber Music Society of Lincoln Center, which is 2 years is jinior.

The New York group plays substantial amounts of 19th-century music and operates on a larger scale. But with imprimateur of the Kennedy Center, the TCP hopes to expand steadily its scale (this season each program at the Center will be repeated, a departure from the past).

Serkin's trip to New York had a measure of paternalism. For the TCP, like numerous other groups, is in part a child of Serkin's celebrated summer festival at Marlboro, Vt., where Pablo Casals conducted and taught during his last years. At least nine of the group's principal players either play at Marlboro or have done so.

Also, the diversity of program communations at Marlboro has greatly influenced groups like the Chamber Players. In this case, the major differences are that TCP has no orchestra and plays less new music.

Discussing such groups, Serkin speaks of "the tremendous expansion of chamber music in this country recently. Why, when I first came hero in the 2930s there were even very few string quartets, much less organizations like these."

But when it is suggested that Serkin and Marlboro have provided the catalyst for much of this activity, Serkin gently denies it. "Oh, I cannot take any credit. It also there is the intense musicality of the people."

Ben Dunham, executive director of Chamber Music America, supports the extent of the growth. "We have no exact figures yet, but we believe there are more than 1,000 professional-level ensembles. Why, we have been in business only since March, and we already have 120 member groups."

But on another point Dunham differs with Serkin. "He is too modest about himself. No person has done more than he to bring this about. And Marlboro has literally spread musicians and ensembles in all directions."

The Theater chamber players literally began at Marlboro. "I had been going there for several years," says Koston, "and then in about 1964 or 1965 Pina Carmirelli and I played in the Brahms Bmajor Trio. To my knowledge it was the first time any two members of the group had played together - except, of course, for my studies with Leon." Fleisher teaches at Baltimore's Peabody Conservatory.

Within a year or two planning for the group was underway.

"The day Dina broached the possibility it took me completely by surprise. Frankly it was the key to a new life for me because I had never conducted before. It had never entered my mind and it seemed far out, even though I don't know why I had never thought of conducting, particularly in those years just beforehand." The latter is an indirect reference to the mysterious physical disability to the right hand that has curtailed Fleisher's spectacular piano career since the early 1960s.

"I first raised the subject at lunch at the Mt. Vernon Restaurant in Baltimore," Koston recalls. "We walked around that afternoon and talked about what we wanted to do. Washington didn't need more of the Beethoven; Schumann and Brahms masterpieces; the Library of Congress was doing that beautifully. Also, it didn't need more all-contemporary ensembles. Anyway, I think that's the wrong approach, because the ear cannot adequately take in a full evening of nothing but new works.

"So we decided on the formula. We still had to come up with the money and a home. The latter was found by the strangest accident. I was marching in a picket line of Artistsof Conscious, an antiwar group, one afternoon and ran into Hazel Wentworth, who was then running the Washington Theater Club at its early location on O Street.

"'Why not use the theater when it's dark on Monday night?", she said. Well, that problem was solved. And Hazel also got us started by helping us get $1,000 from the Meyer Foundation. Thus the group began in June of 1968." Other major supporters have been the National Endowment for the Arts and the Cafritz Foundation.

The budget has risen from $11,987 in 1969-70 to $62,299 last session. Grants constitute about two-thirds of the Chamber Players' budget as expansion speeds up.There is already a CBS "Camera 3" television program in the can. Touring will be extended. Their Carnegie Hall debut is next year, and a New York series is planned for the year after.

Yet the financial footing remains precarious. None of the players are salaried, though the fees are up from $100 to $250. As for Fleisher and Koston, "We never accept a cent until we are sure everybody else is fully paid," he said.

Yet the turnover rate for 11 years has been practically nonexistent. Players, now 32 in number, who have moved out of town continue to participate, and to stay over at Koston's house when here. And after rehearsals and concerts Koston says their favorite way to let off steam is to gather for feasts at the Golden Palace in Chinatown.

Serkin recognizes this camaraderie among chamber players, which is not always present amid the tensions of opera companies and sympthony orchestras. "I can't really explain it," he said, "but have you ever been to Marlboro? Because that is the way you would get the answer to your question. Also, and this is very important, you must come for pleasure, not for work."