Two crops of primary lung tumors in BALB/c mice after a single transplacental exposure to urethane.

Transplacental exposure of BALB/c mice to urethane on day 17 of gestation resulted in the appearance of 2 distinct crops of primary lung tumors (adenomas). The first, smaller crop occurred in 6--12% of the offspring by 12 weeks of age. The second crop, seen after 9 months of age, consisted of tumors in 45% of the offspring. This finding indicates a potentially useful model system for studying tumor latency. It also suggests caution in the choice of short endpoints in time carcinogenesis assays based on lung tumors in certain strains of mouse.