That Agatha Christie's "The Mousetrap" has been running in London for 30 years should not be the most important fact about it, but it is impossible to ignore. Once a play has been running that long, it passes out of the realm of theater and becomes something else--it has been called an institution, but it also could be labeled a living fossil.

The Barter Theater, which opened a production of the play Wednesday, has produced "The Mousetrap" three times previously, each time directed by the same woman, Dorothy Marie Robinson. Although one might question how many times a director can move actors around the same set with enthusiasm, one also must ask whether "The Mousetrap" deserves these endless revivals.

The sad truth is that "The Mousetrap," for all its longevity, is nothing more than an average suspense thriller. It is not nearly as surprising, for example, as "Deathtrap," or as interesting as Christie's "Witness for the Prosecution." Once the revelation of the mystery killer unfolds, after routine foundation-laying, the play collapses and limps quickly to the final curtain.

The preliminaries, set in the agreeably familiar English manor house, include the inevitable assemblage of guests presumably unknown and unrelated to each other. A murder has been committed before the curtain rises, another by the end of the first act. The second act is devoted to unraveling the hidden connections, revealed in a series of traumatic confessions.

The Barter's production is a small step beyond its usual summer stock competence, with a number of actors who handle the basically cardboard characters with professional conviction. Fortunately the weakest member of the cast is killed at the end of the first act. Alexandra O'Karma is tall and elegant as the mysterious Leslie Casewell, and Kevin Spacey, who was so tormented as Liv Ullmann's son in last year's "Ghosts" at the Kennedy Center, is a strong and aggressive Sgt. Trotter. Colin Bruce adds authenticity to the Britishness of the production, aided by Susan Pellegrino as his tweedy young wife. Paul Mackley is a diversion as the eccentric young Christopher Wren.

The notion of keeping "The Mousetrap" alive as a tradition for new generations to see, however, should be buried. Affection for a fine piece of theater is one thing; reviving an unremarkable play is a waste of time, talent and audience.

"The Mousetrap," by Agatha Christie, directed by Dorothy Marie Robinson, set by Bennet Averyt, costumes by Sigrid Insull, lighting by Al Oster. With Susan Pellegrino, Colin Bruce, Paul Mackley, Marlene Bryan, Charles Hudson, Alexandra O'Karma, Bob Horen and Kevin Spacey. At the Harris Theater at George Mason University through Jan. 23.