Interleukin-1 (IL-1) is probably an important lymphokine mediator of inflammation and bone resorption. IL-1 derived from mononuclear cells, a melanoma cell line (MM96 cells), and recombinant human IL-1 (rHuIL-1 beta) increased in vitro bone resorption, as measured by the release of 45Ca from cultured mouse calvariae. The 50% maximum active resorption was observed with 0.125 ng/ml or approximately 10(-11) M rHuIL-1 beta. The resorptive action of IL-1 was not entirely dependent on prostaglandin mediation, since its effect was evident when prostaglandin synthesis was inhibited in the cultures by indomethacin. IL-1-induced resorption has been shown to be inhibited by 10(-5) M 3-amino-1-hydroxypropylidene-1-1-bisphosphonate (APD). This inhibition was partially reversed by increasing doses of IL-1. In vitro toxicity studies showed that at concentrations of 10(-4) M, APD inhibited the growth of cultured MM96, murine myelomonocytic P388D1, and rat osteosarcoma UMR 106 cells, but not other mast and lymphoid cell lines. These in vitro observations may have relevance to the use of APD in bone and joint diseases in which inflammation and bone resorption are prominent.