It's ironic that an entirely non-professional "Romeo and Juleit" turns out to be the best theater Ellie Chamberlain's Shakespeare Summer Festival has produced in some years.

This is free Shakespeare at the Monument Grounds' Sylvan Theater and though the small government funding was increased some this year, it wasn't enough to pay Equity or comparable professional salaries. Robert Peterson, president of the Greater Washington Central Labor Council, has raised the question about under-cutting union scales, but it is a moot one, for this production must end Sunday night.

Now raised, however, the question bears brooding about for future years. In this case, director Roger Meersman's acute awareness of the play's values - its youthfulness, narrative power and this production's Romeo, Lanny Thomas - uses a non-professional cast most effectively.

Apart from the cancellation of last Saturday's performance because of an auto accident involving its Nurse, Leah Kremer, the production has attracted good audiences but, even at the modest fees paid the company, the funding runs out Sundays.

Mary Martin has torn a cartilage in her right knee, but is determined that will not interfere with her appearances in "Do You Turn Somersaults," starting a six-week run at the Eisenhower Theater Aug. 17. The romatic comedy by Soviet playright Aleksei Arbuzov is in rehearsal at the University of Tennessee's Clarence Brown Theater, Knoxville, where it will run next week after previews there this weekend. The University physician, who also looks after its football team, put the injured Martin Knee in a cast yesterday, but Martin declares it will not keep her from performing.

That curtain donated by Japan to serve the Kennedy Center Opera House will reveal a fitting attraction the week of Sept. 6 the Kabuki of the National Theater of Japan. Stemming from the older forms of Noh and Bunraku. Kabuki, at 400 years, is the most recent of the nation's three major theatrical forms. The program wil include "Yashitsune Senbon Zakura," first performed in 1747 and "Korozuka," introduced in 1939. The company's four week American tour its first visit in eight years, will be headed by Ichikawa Ennosuke now his country's leading Kabuki actor.

A new "Spots" is out. This paperbound book ($3) includes floor plans of area theaters and sports arenas, notes about community theater organizations and their phone numbers. At the back is a series of coupons for such attractions and sports events, offering one free admission with the purchase of another. The post office address is Box 1122, Rockville, Md., 20850.

Inspired by Vachel Lindsay's "the Congo" a new musical, "Tapestry" conceived by Harry Poe of Ebony Improptu, is nex for the free, unreviewed Musical Lab Theater, to begin performances Aug. 9 on the Kennedy Center Terrace, Tuesdays through Saturdays at 8 p.m. Tickets are available now at the Friends at the Kennedy Center desk in the Hall of Strikes. The work has been "recommended" by the National Commission to Expand the Scope and Constiutency of Black Participation at the Kennedy Center fro the Performing Arts.

"Sganarelle" is the source of "The Imaginary Cuckold of the Old West," a variation on Moliere's one-actor by Georgetown Classical Theater, to be played Aug. 11-13 and 18-20 at Stage Three. Poulton Hall, 37th and P Streets NW. Moliere's old role is now called Kid Sganarelle; details at 625-3271.

Chicago's Good Theater is determined to survive. The Chicago Theater Group has assumed responsibility to match a $200,000 National Endowment for the Arts grant on a three-to-one basis. Two productions already are set for the coming season, Lynn Redgrave as "Saint Joan," opening Sept. 29, and, in cooperation with Washington's Arena Stage, Stephen Schwartz's musical adaptation of Studs Terkel's "Working," Stanley M. Freehling heads the CTG board, hoping to raise the necessary $600,000 by Dec. 31.

It's been a good year for veteran producer Cheryl Crawford. Besides her memoir, "One Naked Individual," this leader of Theater Guild and Group Theater productions and producer of "Brigadoon," is co-sponsor of "Do You Turn Somersaults?" which will star Mary Martin and Anthony Quay be opening Aug. 18 at the Eisenhower . . . Zero Mostel and company have begun rehearsals for "The Merchant," which follows "Somersaults," Arnold Wesker's drama is a variation on "The Merchant of Venice," which, Mostel says, "I had to read in school." . . . Streets 70's "Hello Dolly?" continues at Rockville Civic Center with Betty Clark in the title role: details at 468-4172 . . . Wildwood Summer Theater continues "Carousel" through Saturday at Bradley Hills Presbyterian Church. Bethesda, details at 365-0410 . . . Saturday night at Wolf Trap. Anna Moffo and John Reardon have the leads in a concert version of "The Merry Widow," with Franz Allers conduction The Filene Center Orchestra and Chorus.