Chemicals classified by the IARC to its Groups 1, 2A, 2B and 3 were examined in an attempt to identify characteristics of their behaviour in experimental studies of carcinogenicity, genotoxicity and acute, mammalian toxicity that correlate with those categories. Only those agents for which carcinogenic potency information was available were studied. For both mice and rats, greater proportions of chemicals were potent carcinogens if they had been categorized in Group 1 (human carcinogens) than if they had been put into one of the other categories. Not surprisingly, there was a weak association between carcinogenic potency and acute toxicity. Mice were especially sensitive to tumour induction by halides, while the lower sensitivity of rats to any carcinogenic effect of halides could be due in part to the higher systemic toxicity of halides in this species: a reduced differential of toxic and carcinogenic doses decreases the dose window in which carcinogenic effects may be demonstrated. It was notable that the human carcinogens were active in those genotoxicity tests with higher specificity for identifying rodent carcinogens. Predictive assays for carcinogenicity considered to have high specificity were in vivo cytogenetic, hepatocyte unscheduled DNA synthesis and Salmonella (5 commonly used strains) and mammalian cell hprt locus mutation assays. None of the relationships was strong enough to form the basis of a simple categorization process, but they could serve to alert investigators to chemicals of special toxicological interest and importance.