The fungal metabolite FK506 was discovered because it shared an important property, the ability to inhibit production of IL-2, with another well-known immunosuppressive fungal metabolite, CsA. FK506 has, since its isolation, also been shown to share other immunosuppressive effects with CsA. This study was performed to further investigate the in vitro immunological properties of FK506, in comparison with and in combination with CsA, to evaluate the plausibility that their mechanisms of action were identical or similar, in spite of their different molecular structures. The ability to inhibit several responses of human peripheral blood lymphocytes to mitogenic and alloantigenic stimulation was explored. Synergistic effects of the two drugs were extensively studied, since this would provide additional information regarding their mechanisms of action. Also, similar immunosuppressive properties would enable the use of the two drugs in combination. We found that FK506 and CsA had very similar mechanisms of action and additive effects were recorded.