Benzene is a widespread human carcinogen, inducing leukaemia and hematotoxicity. It has been shown to be a multi-organ carcinogen in animals. The effect of benzene was studied using induction of micronuclei (MN) in whole blood lymphocytes cultures after treatment with different concentrations of benzene (5, 10, 50, 100, 500 and 1000 microM) with and without metabolic activation (S-9 mix). A significant elevation in the induction of micronuclei was found after application of benzene at doses of 50 and 100microM in both donors. Treatment of bovine lymphocytes did not result in the induction of micronuclei in a dose-dependent manner. The addition of an external metabolic factor (10 % S-9 mix for 2 h) in blood cultures treated with benzene indicated an increase of the genotoxic activity of benzene (at concentrations ranging from 10-100 microM).