The regulatory effect of prostaglandin (PG) E2 and a cyclooxygenase (COX) inhibitor on Bacille Calmette–Guérin (BCG)-induced macrophage cytotoxicity in a bladder cancer cell, MBT-2, was studied in vitro. BCG stimulated thioglycollate-elicited murine peritoneal exudate cells (PEC) to induce cytotoxic activity and to produce cytokines such as interferon (IFN)-γ, tumour necrosis factor (TNF)-α and PGE2. NS398, a specific COX-2 inhibitor, and indomethacin (IM), a COX-1 and COX-2 inhibitor, enhanced viable BCG-induced cytotoxic activity and IFN-γ and TNF-α production of PEC. However, NS398 and IM did not enhance these activities induced by killed BCG. Enhanced cytotoxicity was mediated by increased amounts of IFN-γ and TNF-α. Exogenous PGE2 reduced cytotoxic activity and IFN-γ and TNF-α production of PEC. These results suggest that PGE2 produced by BCG-activated macrophages has a negative regulatory effect on the cytotoxic activity of macrophages. Accordingly, a PG synthesis inhibitor may be a useful agent to enhance BCG-induced antitumour activity of macrophages.