The sensitivity of human bronchial epithelial cells to induction of micronuclei was determined in cultures derived from different donors. Two direct-acting carcinogens, (+ or -)-7,8 -dihydroxy-9,10-epoxy-7,8,9,10- tetrahydrobenzo (a)- pyrene (BPDE) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were used to induce micronuclei. Both agents increased the incidence of micronuclei in a concentration-dependent fashion, even at concentrations which did not produce cytotoxicity. Cytokinesis was blocked with cytochalasin B so that micronuclei were counted only in binucleate cells, thereby decreasing the total number of cells that need to be examined and also eliminating variations due to possible differences in cell growth rates. The results demonstrate the potential usefulness of the micronucleus assay as a sensitive measure of genetic damage in human epithelial cells. (Copyright (c) 1990 Pergamon Press plc.)