Fumonisin B1 (FB1) is a mycotoxin produced by Fusarium verticillioides, a fungus that commonly contaminates maize. In the present study, we investigated the effects of FB1 on swine peripheral blood mononuclear cells (PBMC) by measuring cell proliferation, cell cycle progression and interleukin (IL)-2 production. Forty-eight hours after treatment in vitro, FB1 induced a decrease of PBMC proliferation as measured by cell counting and dehydrogenase enzyme activity. This effect was observed starting with 10 microM FB1. The effect of FB1 on cell cycle progression was analyzed by flow cytometry. Incubation of PBMC with FB1 increased the percentage of cells blocked in G0/G1 phase of the cell cycle. Treatment with 200 microM FB1 induces a high blockade of the cell cycle, with 92.4% of cells in G0/G1 phase. This blockade was observed in all lymphocyte subsets tested (CD2+, CD4+, CD8+ and Ig+) as evidenced by dual staining for DNA and membrane surface molecules. A significant decrease of IL2 production was also observed in the supernatants of ConA stimulated PBMC treated with 100 or 200 microM FB1. In conclusion, these data suggest that FB1 may affect immune functions by inhibiting lymphocyte proliferation and IL2 production, suggesting a possible role of FB1 exposure during infectious disease and cancer.