The techniques for culturing erythroid precursors made possible the study of the effect of steroids on these cells, and it has been well established that androgens and 5β-steroids have a direct effect on erythroid precursor cells from animal or human bone marrow. By contrast, their mechanisms of intracellular action remain poorly understood. We used tritiated methyltrienolone (R1881), a synthetic androgen that binds strongly to the androgen receptor, to characterize the binding activity in nuclear extracts of erythroblasts from human bone marrow cultures. The nuclear extracts contained binding sites that were saturable at low concentrations of <sup>3</sup>H-R1881 (8-12 n M). Scatchard analysis revealed that the dissociation constant of the hormone-receptor complexes (K<sub>d</sub>) was 10-20 n M, and the number of binding sites was 64-103 fmol/mg of protein. On linear sucrose density gradient analysis (5-20%), the hormone-receptor complexes sedimented in the region of 3.9 S. Finally, 5β-dihydrotestosterone had also a strong affinity for the binding sites. The nuclear component binding has all the physicochemical characteristics usually attributed to androgen receptors. These data strongly suggest that androgen action on erythropoiesis is mediated by a nuclear androgen receptor.