Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristic of the original transformed parent.Although there is no controversy over the fact that mutagenic carcinogens can malignantly transform mammalian cells in vitro and in vivo (8,23) or over the fact that point mutations can be shown to accompany this transformation (30), whether mutations are necessary to malignantly transform a cell with a chemical carcinogen is still widely debated (11). One reason the role of mutations has been so difficult to assess is that in almost all transformation systems studied, if the endpoint is a cell capable of malignant growth, the process of cell transformation is the result of the sum of at least two steps (4,5,19,31,43). Sorting out the contribution of each step and the mechanism underlying each is only just beginning.There is however one cell system in which transformation can be induced to occur in a single step: the baby hamster kidney cell line BHK-21 clone 13 (BHK-21/cl 13) (9, 17, 51). This permanent cell line behaves as though the process of promotion or progression has already taken place. Only one additional event, the induction of anchorage independence, is needed to change BHK cells from nontumorigenic to tumorigenic (17); therefore, this transformation can be analyzed clearly and quantitatively. In experiments in which chemical carcinogens were used on a subclone of BHK cells (9), the frequency of spontaneous transformation, induced transformation, and reversion to normal and the occurrence of temperature-restricted transformants suggest that transformation is due to a somatic mutation. This paper provides more direct evidence for this assertion by demonstrating that mutation and transformation occur with parallel kinetics in response to increasing doses of carcinogens and that the type of transformants which are induced depend upon the type of mutations which the inducing carcinogen is capable of p...