To evaluate the frequency of micronucleated lymphocytes in a normal population, 45 healthy subjects were analysed by using a modified cytochalasin B block method; the influence of some confounding factors (sex, age, smoking, etc.) were taken into account. Using a stepwise regression test smoking habits were found to have a statistically significant influence on the frequency of micronucleated cells and micronuclei. In addition, the mitotic and proliferative indexes, and the frequency of micronucleated lymphocytes, at different culture times, were studied in four healthy subjects. Based on the results we suggest that cytochalasin B be added to cultures no later than the 42nd hour.