A high molecular weight substance is described which has been obtained from crude rat testis Leydig cell membrane fractions by exposure to buffer systems of low ionic strength. This material binds human [125I]iodo-CG in an apparently specific way. The binding of labeled human (h)CG to this soluble binding component (SBC) is inhibited by an excess of unlabeled hCG, but not by FSH or prolactin. The sedimentation coefficient of the complex was determined to be 4.35 S when the complex was formed after SBC had been solubilized, and 5.0 S when the complex had been formed with the particulate receptor before solubilization. The mol wt of the human [125I]iodo-CG-SBC complex was calculated from Kav-values and sedimentation coefficients and found to be 71,500 (4.35 S) or 82,250 (5.0 S), respectively. In a modified receptor assay (RRA), uptake of tracer hormone by membrane-bound particulate receptors was inhibited by SBC. This, together with chromatographic studies on Sephadex G-200 and on Sepharose 6B columns, indi...