—I was most interested in your report of cimetidine-induced potentiation of carmustine and cranial irradiation. We have recently investigated the interaction of cimetidine augmentation of cyclophosphamide in mice. The results1 showed substantial increases (near doubling) in the antitumor activity of cyclophosphamide in mice given concomitant cimetidine therapy. We have previously seen cimetidine augmentation of theophylline levels in man and, taken together, the mechanism in both situations is most probably cimetidine blockade of cytochrome P450 metabolism. Since carmustine is extensively metabolized by P450 mixed-function oxidase systems, your findings of a notable interaction are consonant with this explanation. Your timely report on this interaction should have immediate relevance to the practicing oncologist.