The biological effects of in vitro exposure of cultured fetal hamster tracheal epithelium to benzo(a)pyrene in 0.5% dimethylsulphoxide (BaP: 5 and 10 micrograms/ml medium) were studied by light and scanning electron microscopy. The solvent, DMSO, alone induced a slight, temporary retardation in development and differentiation of cilia. At day 28 of culture, however, differences in growth and maturity disappeared between DMSO-exposed and untreated explants. No dose-related differences found in the surface morphology of alterations were seen in BaP-treated tracheae; the explants revealed metaplastic foci and reduced, incomplete ciliogenesis compared to DMSO- and non-treated controls. Towards the end of the culture period, ciliogenesis tended slowly to replace epithelial alterations.