The novel immunosuppressive drug FK-506 inhibits both cell-mediated and humoral immunity in vivo and suppresses T-lymphocyte activation at nanamolar concentrations in vitro. We report here that FK-506 also inhibits the in vitro proliferation of murine B lymphocytes induced by goat antimouse IgM (GaMIgM), although it does not affect the induction of proliferation by bacterial lipopolysaccharide (LPS). The action of FK-506 on B-lymphocyte proliferation thus resembles that of cyclosporin A, further strengthening the correlation between the action of the two immunosuppressive drugs previously reported for T-lymphocyte activation.