A high-affinity, low-capacity androgen binding component in human placental cytosol was characterized. (3H)-R1881, a synthetic androgen, was used as radioactive ligand for characterization and quantification of this placental androgen binding component. Various naturally occurring as well as synthetic steroids were first investigated for their in vitro stability under receptor assay conditions (i.e., an overnight incubation at 4 degrees C). Only the synthetic steroids were found to be metabolically stable. The rate constant of dissociation was found to be 6.42 X 10(-4) s-1 (on the basis of k-1 the rate constant of association was 1.27 X 10(5) M-1 s-1). The concentration of androgen binding protein was measured in 50 human term placentae. The mean concentration was 169 +/- 132 fmol/mg protein, and the average Kd was 5.07 +/- 2.2 X 10(-9) M. Placentae of female fetuses contained significantly higher concentrations (219 +/- 140 fmoles/mg protein) than those of male fetuses (115.2 +/- 101 fmoles/mg protein). The (3H)-R1881-androgen binding protein complex exhibited a sedimentation coefficient of 4S under low salt conditions. The binding site was only specific for androgens and progestins but not for steroids of other classes.