Summary The effects of Fusarium extract and its principal components (T2-toxin, di-acetoxyscirpenol and Butenolide) on the immune system were tested in mice. Animals were treated with these mycotox-ins, and the mitogen responsiveness of spleen or thymic cells was examined. The stimulation of both T and B cells was found to be reversibly inhibited. Also, the ability to synthesize anti-SRBC antibodies was reversibly suppressed. The direct effect of mycotoxins was tested in vitro in lymphocyte and fibrosarcoma cell cultures. These compounds exerted a direct cytostatic effect at high concentrations, and a stimulating effect at low concentration. Profound histological changes were observed in thymus and spleen after treatment, while under the experimental conditions employed, the histology of other organ systems was not affected.