Fumonisins have been reported to have diverse effects on animals, including immunosuppression in chickens and feeder calves; therefore, the effects of fumonisin B1 (FB1) on immune function in BALB/c mice was investigated. When administered i.p. with sheep red blood cells (SRBC), 5 to 100 micrograms of FB1 reduced the number of plaque-forming cells (PFC) produced against SRBC; however, when administered daily, 1 to 50 micrograms of FB1 caused a 4 to 12-fold increase in the number of PFC after SRBC injection. Therefore, FB1 is not only immunosuppressive; but also, immunostimulatory. To test the possibility that there may have been an immune response to FB1 as an antigen, FB1 was injected into mice and the number of splenic cells forming rosettes on FB1-treated SRBC was determined. There were dose-dependent increases in the antigen-binding cells, with up to 4.9- and 4.6-fold increases, respectively, upon primary and secondary immunization. FB1-binding immunoglobulins could be detected in sera from some treated mice, but this response was not obtained in every experiment. In summary, these results show that FB1 has diverse effects on the immune system, causing both stimulation and suppression of the response to foreign antigens, and apparently inducing an antigenic response to FB1.