Defined methods to grow replicative cultures of normal human bronchial epithelial cells either without serum or Swiss mouse 3T3 feeder-cells have been developed. These cells can be subcultured several times, will undergo 35 population doublings, and have expected epithelial cell characteristics of keratin, desmosomes, and blood group antigens. Further, mitotically quiescent cells will differentiate into cells with beating cilia, a characteristic of normal bronchial epithelium. Dissociated single cells form colonies when plated at low density. In vitro carcinogenesis experiments with normal bronchial epithelial tissue and cell cultures have yielded population of cells which have abnormal characteristics. These phenotypically altered cells (PACs), which have keratin epithelial cell markers, have extended population doubling potentials, normal and abnormal human karyologies and abnormal differentiation control. Amosite asbestos was found to be 100 times more cytotoxic for bronchial epithelial cells compared to bronchial fibroblastic cells. In addition, focal hyperplasia and epidermoid metaplasia were observed in explants of human bronchial tissue two weeks after a single exposure to amosite asbestos; both intracytoplamic and intranuclear asbestos fibers were seen by X-ray microanalysis in these lesions.