Deoxynivalenol (DON), 3-acetyldeoxynivalenol (acetylDON) and zearalenone (Ze) were examined for their in vitro effect on mitogen-induced lymphocyte blastogenesis using rat or human peripheral blood lymphocytes (PBL). A dose-dependent reduction of lymphocyte proliferation was demonstrated for each mycotoxin. However, the inhibitory effect of DON was significantly higher than that of the acetylated compound. Concentrations of 90 ng/ml and 220 ng/ml inhibited rat and human lymphocyte blastogenesis by 50%, respectively, whereas 450 ng/ml and 1060 ng/ml acetylDON were required to produce the same effect. The amount of Ze necessary to inhibit blastogenesis by 50% was 250 times greater than required for DON. There was no evidence of cell death and combinations of DON and Ze did not alter the expected response.