Buffer (0.05 M Tris, pH 7.5) soluble factors capable of specifically binding radioiodinated human follitropin ('251-hF!3H) were detected in high speed supernatants (cytosol) of calf testes homogenates. This '2SI-hFSH binding activity, designated as the buffer-soluble binding component (BSBC), did not sediment at 250,000 X g, passed a 0.45 p filter, and was equally distributed throughout the tis...