''I don't know what's meant by love - I never have,'' confesses James, the suddenly unfaithful husband in the bleak new tragicomedy at the Longacre Theatre. The admission probably comes as close as anything to explaining the malaise that underlies ''Passion,'' by Britain's Peter Nichols.
The play concerns the obsession of a middle-aged restorer of paintings with an aggressive young seductress. James's wife is driven to attempt suicide by knowledge of the affair, while its continuance irreparably damages his 25-year marriage.
To reveal the married couple's inner thoughts, the author provides each spouse with an alter ego: James/Jim (Bob Gunton/Frank Langella) and Eleanor/Nell (Cathryn Damon/E. Katherine Kerr). The device is good for some initial amusement but grows tedious and even confusing as the confrontations and exchanges become more complex.
Eleanor's activities as a choral singer are referred to by introducing recorded bits and pieces of Mozart, Beethoven, and particularly Bach's ''St. Matthew Passion'' - intimations of a sublimer love. A portrait of Jesus given James to restore further symbolizes the Christianity the couple have rejected. (The rueful comedy was called ''Passion Play'' when originally presented by the Royal Shakespeare Theatre.)
An estimable Broadway cast includes Roxanne Hart as the promiscuous home-wrecker and Stephanie Gordon as one of her most recent victims. The players do what they can to create a climate of understanding for the rootless human beings caught up in this painful, protracted, explicit case history.
The production was directed by Marshall W. Mason, with an ingeniously adaptable set by John Lee Beatty, costumes by Jennifer von Mayrhauser, and lighting by Ron Wallace.