A chemiluminescence (CL) was observed immediately after the addition of luminol to thioglycollate-elicited ICR mouse peritoneal macrophages (M phi) that had been incubated overnight with recombinant murine interferon-gamma (IFN-gamma) and lipopolysaccharides (LPS). The intensity of this CL was closely correlated with the cytotoxic activity of M phi. NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthase, inhibited the induction of this CL, and L-arginine restored the L-NMMA-induced inhibition. These results suggest that NO is directly involved in the induction of CL. However, we found that immune complexes are required for the induction of CL as well as NO.