Selected direct-acting and metabolism-dependent compounds are administered by various routes to non-pregnant and to precisely timed-pregnant rodents and non-human primates, and the treated animals and their offspring are followed for subsequent tumor development. Neoplasms which develop are classified by light and electron microscopy, histochemistry, transplantation, and cultivation in vitro, and dose/response relationships are precisely quantitated to establish age-dependent qualitative and quantitative differences in susceptibility to different types of carcinogens in different organ systems.