This study provides a preliminary comparative evaluation of the responses to a series of 49 chemicals, in in vitro transformation assays, of Balb/3T3 cells, Syrian hamster embryo cells, and Fischer 344 rat embryo cells infected with Rauscher murine leukemia virus. The chemicals assayed included aromatic amines; polycyclic aromatic hydrocarbons; alkylating agents; nitrosamines, hydrazines, and related compounds; heterocyclic compounds; amides, ureas, and acylating agents; inorganic compounds; and hormones. In all three assays 37 of the chemicals were tested. The most uniform test responses were obtained with the polycyclic aromatic hydrocarbons and inorganic compounds With the other groups of chemicals, more variation in response was observed. This study expands the base of information on the potential of these in vitro transformation systems, and the lack of responses with some of the chemicals underscores the need for incorporation of exogenous metabolic activating systems into these assay systems.