Toward a classification scheme for degrees of experimental evidence for the carcinogenicity of chemicals for animals.

A classification scheme is proposed for degrees of experimental evidence for the carcinogenicity of chemicals for animals. The classification stems from the suggestions of an IARC Working Group that the evaluation of bioassays include consideration of whether an increase in malignant tumours occurred and whether it occurred to an unusual degree or in multiple experiments. We extended the evaluative process to chemical experiments with no evidence of carcinogenicity and gave increased emphasis to results from more than one animal species. Although the proposed classification was developed for a group of NCI bioassays which were similar in design and conduct, it may provide a general framework for the evaluation of carcinogenesis bioassays and stimulate development and application of evaluative methods.