An assay was developed and measurements of aryl hydrocarbon [benzo(a)pyrene] hydroxylase were made in peripheral blood monocytes from a human population. Treatment with the carcinogen benz(a)anthracene in cell culture increased aryl hydrocarbon hydroxylase activity from 6.5- to 37-fold in monocytes from each of 25 apparently healthy donors. A weak correlation (r = 0.38) was observed between the induction ratios obtained with monocytes and lymphocytes from the same donors. Reproducibilities of the monocyte and lymphocyte assays were comparable. In monocytes, the measurement of basal and induced aryl hydrocarbon hydroxylase activity does not require pretreatment with mitogens as is the case with lymphocytes. Monocytes also exhibit a wider range of induction ratios than lymphocytes.