The effects of chemical carcinogens on various aspects of cellular metabolism were measured in human IMR‐90 fibroblasts. These studies were undertaken to assess the ability of IMR‐90 cells to be used for the study of chemical carcinogenesis in vitro. It was found that (1) chemical carcinogens inhibited DNA synthesis in synchronous cultures of IMR‐90 fibroblasts; (2) the effective concentrations of carcinogens showing effects in this system were lower than for other published in vitro cell assay systems; (3) compounds that require metabolic activation to a carcinogenic form demonstrated the ability to inhibit DNA synthesis in IMR‐90 fibroblasts; (4) a fluorometric determination of aryl hydrocarbon hydroxylase (AHH) activity in IMR‐90 cells revealed a low level of inducible AHH; (5) IMR‐90 cells can metabolize 7,12‐dimethyl‐benzanthracene (DMBA) to compounds that are identified as metabolites of DMBA in vivo.