Fumonisin B1 (FB1), a mycotoxin produced by Fusarium moniliforme and related fungi, is nephrotoxic, neurotoxic, hepatotoxic, carcinogenic and immunosuppressive in animals and man. In this study we evaluate the modifications of fluidity, endocytosis and peroxidative damage of plasma membrane induced by FB1 in macrophage cell line J774A.1. In these immune cells FB1 (1-10 microM) enhances membrane fluidity and increases, time-dependently, the horseradish peroxidase (HRP) endocytosis. This effect is concentration-dependent, significant at 10 microM, and reverted by IFN-gamma (100 U/ml). Moreover, FB1 (1-10 microM) induces a membrane peroxidative damage as evident by the increase of malondialdehyde (MDA) production. All these mycotoxin effects provide additional insight into potential mechanism by which FB1, in macrophages, might enhance membrane damage and oxidative stress contributing to the pathogenesis of mycotoxin induced diseases.