The macrolide antibiotic FK-506 causes a strong inhibition of human T and B cell proliferation in vitro in concentrations 100-fold lower than cyclosporine (CsA). However, B cell activation appears to be less sensitive to the immunosuppressive action of FK-506 than T cell responses. There was no indication of a synergistic interaction of the two agents. In contrast, CsA added together with FK could counteract the immunosuppressive activity of the latter drug.