As an important adjunct to the testing of crude natural materials or defined chemicals for their inhibitory capacities, studies of the effects of test substances on carcinogen metabolism will be carried out. This will serve several purposes. In the case of crude materials, it will provide a potential assay for use in isolating and identifying the inhibitor. For both crude and defined test substances it may provide corrobative evidence for in vivo inhibition and also might provide some information on mechanism of inhibition. In some instances, it may detect weat or unusual effects that merit further exploration. Ultimately the metabolism studies may be developed into preliminary screening techniques useful in selecting materials for study in full animal carcinogeesis testing.