Aflatoxin B, (AFB1), a potent hepatocarcinogen, has been known to impair non-specific and specific immune responses. Nitric oxide (NO), hydrogen peroxide (H2O2), superoxide anion (O2-) and tumor necrosis factor-alpha (TNF-alpha) produced by macrophages play an important role in host defense against tumors and microorganisms. In the present studies, we investigated the involvement of those products in the reduced antitumor activities by AFB1. When macrophages are stimulated with LPS after AFB1-pretreatment, the cytolytic activities decrease in a dose-dependent manner. The addition of N(G)-monomethyl arginine (NMMA), anti-TNF-alpha antibodies, catalase and peroxidase decreases antitumor activities further. In contrast, superoxide dismutase (SOD) does not change the antitumor activities. NO and TNF-alpha production was reduced by the addition of NMMA and anti-TNF-alpha antibodies, respectively. Taken together, these data indicate that the reduced antitumor activities in murine peritoneal macrophages are mediated by the suppressed production of NO, TNF-alpha and H2O2 by AFB1 pretreatment, suggesting that the inhibitory effect of AFB1 on those materials may provide the tumors with readily growing condition in vivo.