A novel immunosuppressive compound extracted from the fermentation broth of Streptomyces tsukubaensis belongs to the macrolide family. We gave this drug (FK-506) to MRL/lpr and NZB X NZW F1 (B/W F1) mice, an animal model of human systemic lupus erythematosus (SLE), to investigate its effect on the course of the disease. This drug showed potential to prolong the lifespan, to reduce proteinuria, and to prevent the progression of nephropathy. Appreciable differences in the levels of anti-dsDNA antibodies between treated and control animals were nil.