We have studied the effects of the immunosuppressive drug cyclosporin A (CsA) on the generation of inositol 1, 4, 5-trisphosphate (IP 3) and intracellular Ca 2+ levels elicited upon ligation of murine macrophage receptors for α 2-macroglobulin, bradykinin, epidermal growth factor, and platelet-derived growth factor. Preincubation of cells with CsA (500 ng/ml), either alone or with the various ligands, did not inhibit the synthesis of IP 3. However, we observed 70–80% inhibition of the binding of [3 H] IP 3 to IP 3 receptors on macrophage membranes isolated from CsA-treated macrophages. Preincubation of macrophages with CsA abolished IP 3-mediated release of Ca 2+ from intracellular stores and Ca 2+ entry from the extracellular medium observed when macrophage receptors were stimulated with ligands in the absence of CsA. Preincubation of macrophages with CsA also significantly inhibited DNA synthesis induced by ligands for all four receptors studied. Thus in macrophages, as in T cells, CsA blocks receptor-activated signal transmission pathways characterized by an initial increase in intracellular Ca 2+ concentration. This inhibition appears to result from a drug effect on IP 3 receptors.