The sensitivity of human bronchial epithelial cells to induction of micronuclei was determined in cultures derived from seven different donors. Two direct-acting carcinogens, dl-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and N-methyl-N′-nitro-N-nitrosoguanidine were used to induce micronuclei. Both agents increased the incidence of micronuclei in a concentration-dependent fashion in cells from most donors, event at concentrations that did not produce appreciable cytotoxicity; there were considerable variations in the responses of different donors. Cytokinesis was blocked with cytochalasin B so that micronuclei were counted only in binucleate cells, thereby decreasing the total number of cells that needed 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 from the lung.