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Androgen binding sites have been identified in circulating human mononuclear leukocytes of healthy donors of both sexes. Cells were separated from blood samples on a Ficoll gradient and incubated with different concentrations of [ 3 H]testosterone in the presence or absence of a 400-fold excess of unlabelled testosterone. Binding data were derived from Scatchard analysis. The binding sites fulfil the required criteria for specific steroid binding sites however differ somewhat from the classic androgen receptors from genital skin fibroblast: in fertile adult males ( n = 20) the binding sites showed (1) a high affinity for testosterone (1.32 ± 0.49 nM; mean ± SD), (2) a saturable capacity (184 ± 52 binding sites per cell; mean ± SD), and (3) a characteristic competitive binding profile for other steroid hormones (relative binding affinities: testosterone = dihydrotestosterone > 17β-estradiol > progesterone, whereas aldosterone, 17-hydroxy-progesterone and cortisol did not compete appreciably). Furthermore the number of binding sites determined using [ 3 H]dihydrotestosterone, [ 3 H]RU-1881, or [ 3 H]testosterone were comparable. This raises the possibility that androgen receptors in peripheral mononuclear leukocytes differ from those in genital skin fibroblasts. There was no apparent correlation between serum testosterone concentrations and androgen binding sites. In fertile women remarkable changes in androgen binding sites were seen in the course of the menstrual cycle, with a significant increase in the immediate preovulatory period. The presence of androgen receptors in peripheral mononuclear leukocytes provides for the first time the experimental basis for an hypothesis of direct, receptor-mediated effects of androgens on mature immunocompetent cells. The immunological implications of these results are discussed.