We studied the effect of an inhibitor of nitric oxide (NO) synthesis, NG-monomethyl-L-arginine (LNMMA), on the Bacillus Calmette-Guérin (BCG)-induced antitumor activity of murine peritoneal exudate cells (PEC) against murine bladder cancer cell line MBT-2 in vitro. L-NMMA enhanced BCG-induced cytotoxic activity of PEC, as well as interferon (IFN)-γ and tumor necrosis factor (TNF)-α production. The L-NMMA-induced enhancement was due to the prolonged survival of BCG in macrophages, because no enhancement of cytotoxicity was observed and neither IFN-γ nor TNF-α production was significantly enhanced by killed BCG. Anti-TNF-α antibody (Ab) and anti-IFN-γAb reduced the L-NMMA-induced enhancement of the cytotoxicity. The depletion of T cells from PEC reduced the production of both IFN-γ and TNF-α, as well as the enhancement of cytotoxicity induced by viable BCG plus L-NMMA. These results suggest that L-NMMA has an enhancing effect on BCG-induced macrophage cytotoxicity and the enhancement is partially mediated by T cells and their soluble products. Accordingly, NO inhibitor should be a valuable adjunct to BCG immunotherapy for bladder cancer.