Liver tumors were induced in rainbow trout (Salmo gairdneri) 1 year after carcinogens were microinjected into embryos. Neoplasms were induced by a single injection of 13 and 25 ng per egg of aflatoxin B1 (CAS: 1162-65-8), 500 ng per egg of 7,12-dimethylbenz[a]anthracene (CAS: 57-97-6), and 250 ng per egg of 2-anthramine (CAS: 613-13-8). Over 70% of [3H]benzo[a]-pyrene (CAS: 50-32-8) injected into eggs was retained in hatched embryos, 120 hours postinjection. Exogenous activation of test compounds with the use of rat liver microsome preparation (S-9) increased the incidence of liver tumors in fish given injections of 25 ng aflatoxin B1. The amount of chemical required for the embryo injection assay was comparable to that required for the Ames bacterial mutagenesis assay.