Rapamycin (RAPA) is a potent immunosuppressant and can effectively prevent allograft rejection at a dosage 10- to 100-fold lower than that of cyclosporin A. RAPA strongly inhibits proliferation and function of T and B cells. In this study, we investigated the effect of RAPA on human HLA-unrestricted cell killing. It was shown that in vitro RAPA inhibited the cytolytic effect of natural killer cells and lymphokine-activated killer cells (LAK) and inhibited antibody-dependent cell-mediated cytotoxicity. The effective concentration of RAPA was 10- to 100-fold higher than that required for inhibiting T cell proliferation. These results suggest that there could be a therapeutic dose window at which RAPA inhibits T cell activity while it leaves HLA-unrestricted cell killing unaffected. We also demonstrated that while IL4 inhibited LAK activity, and RAPA inhibited IL4-promoted T cell proliferation, RAPA was not able to antagonize IL4's inhibitory effect on LAK. This indicates that the mechanism of interaction between RAPA and IL4 on LAK is different from that on T cells.