In order to investigate the relationship between mutation and carcinogenesis in vivo, a method has been developed in which somatic mutation can be studied in specific organs of Chinese hamsters after acute dosing with chemical carcinogens. In preliminary experiments an increase in mutation was detected in cultures derived from lung tissue of hamsters after intraperitoneal dosing with the direct-acting mutagen and weak carcinogen, ethyl methanesulphonate, or with diethylnitrosamine, a carcinogen which requires metabolic activation before mutagenic activity can be demonstrated in vitro. Subsequent experiments have shown the induction of mutation in cells dereived from the bladder of animals dosed with diethylnitrosamine or methyl nitrosourea and in a variety of tissues from animals dosed with methane-sulphonates, 2-acetylaminofluorene and 3-methylcholanthrene.