The inhibitory effect of crude Fusarium poae extracts, T2-toxin and diacetoxyscirpenol on synthesis of antibody to sheep red blood cells was investigated, as well as the effect of T2-toxin on skin graft rejection. Fusarium crude extracts, T2-toxin and diacetoxyscirpenol cause a significant reduction in thymus weight and inhibit responsiveness to sheep red blood cells. The effect on antibody synthesis was critically dependent on the drug doses used. The effect of an immunosuppressive dose of T2-toxin disappeared within 6 days after the end of the treatment. The administration of T2-toxin subsequent to immunization stops antibody synthesis. T2-toxin significantly prolongs the period required for skin graft rejection.