Exposure of SV40-transformed Chinese hamster embryo cells (line CO50) to a series of physical and chemical carcinogens (including activation-dependent and activation-independent varieties) resulted in the induction of viral DNA synthesis. The carcinogen mediated amplification of SV40 DNA was demonstrated by a highly sensitive in situ hybridization procedure for the detection of cells synthesizing SV40 DNA. Treatment of CO50 cells with an inhibitor of polycyclic hydrocarbon metabolism (7,8-benzoflavone) prior to the application of benzo[a]pyrene or 7,12-dimethyl-benz[a]anthracene prevented the induction of SV40 DNA synthesis, indicating that the induction depends upon the metabolic activation of these compounds. Non-carcinogenic hydrocarbons were inactive under this assay. Two different protocols for determining the inducing potential of a compound are presented. The properties of this test and its possible use as a short-term assay for potential carcinogens is discussed. The possibility that the induction of SV40 DNA synthesis is a reflection of a general gene amplification phenomenon mediated by carcinogens is discussed.