When fura-2/AM loaded peritoneal macrophages were treated with cisplatin, it resulted in a rapid increase in the intracellular free calcium. Calcium modulating agents, EGTA, nifedipine, and TMB-8, and the calmodulin antagonist W-7 inhibited cisplatin-induced tumoricidal activity of murine peritoneal macrophages. Supernatants collected from macrophages treated with cisplatin and EGTA, nifedipine, TMB-8 or W-7 demonstrated decreased tumor necrosis factor (TNF) and interleukin-1 (IL-1) activity in comparison to supernatants collected from macrophages treated with cisplatin alone. Similarly, TNF and IL-1 activity were significantly inhibited in paraformaldehyde (PFA)-fixed macrophages and freeze-thaw lysates of macrophages treated with cisplatin and different calcium and calmodulin-modulating agents. These results suggest that calcium and calmodulin are important in the cisplatin-mediated activation of murine peritoneal macrophages to tumoricidal state.