The ability of cyclosporine (CSA) and FK506 to inhibit cytokine production by factor-dependent murine mast cell lines was investigated. The mast cell clone, MC/9, and two mast cell lines, MCIII and MCVI, were stimulated to produce cytokines with phorbol myristate acetate plus the calcium ionophore A23187. The production of cytokines by stimulated mast cells cultured in the presence or absence of drug was monitored by bioassay of culture supernatants for induction of proliferation by factor-dependent cell lines and inhibition of these responses by neutralizing monoclonal antibodies. Both CSA and FK506 inhibited mast cell cytokine production at concentrations comparable to those observed with T cells. However, the degree of inhibition of cytokine production varied among the mast cell lines as well as between different cytokines produced by a given mast cell line. For example, CSA completely inhibited interleukin-2 (IL-2), IL-3, IL-4 and granulocyte-macrophage colony stimulating factor secretion by all three lines, with the exception that IL-2/IL-4 production by MCIII was partially resistant to inhibition by CSA. Similarly, FK506 completely inhibited cytokine production by MC/9, partially inhibited cytokine production by MCIII and had differential effects on IL-3/granulocyte-macrophage colony-stimulating factor and IL-2/IL-4 production by MCVI. Consistent with their ability to selectively inhibit cytokine gene transcription in T cells, neither CSA nor FK506 inhibited factor-dependent proliferation by these mast cell lines. In view of the putative role of cytokines in inflammation and late phase asthmatic reactions, these observations may be of particular significance in development of methods of pharmacologic intervention.