A comparative study was made of transplacental (in vivo) and direct (in vitro) effects of benz(a)pyrene (BP) in the organ cultures of embryonal lungs of mouse susceptible (strain A) and resistant (C57 Bl) to pulmonary blastomogenesis. The development of the epithelium squamous cell metaplasia was observed only after direct application of BP on the lung explants in vitro. Correlation between metaplasia frequency and BP dose and duration of treatment and mouse susceptibility to pulmonary blastomogenesis is established. Thus, at all BP concentrations (3.6 or 12 micrograms/ml of medium) and maximum time of treatment (15 days) the frequency of squamous cell metaplasia in the lung explants of C57 B1 mice was very low (3.0, 2.0 and 4.2%, p > 0.1) while in a strain A it was significant at the doses of 6 and 12 micrograms/ml (10.4 and 23.4%, p < 0.001). No epithelial metaplasia developed when the time of BP treatment was decreased up to 7 days.