The quaternary structural relationships between subunits of the follitropin (FSH) receptor were determined through the use of the reversible, homobifunctional, chemical cross-linking reagent bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone (BSOCOES) after formation of covalent 125I-azidobenzoyl-FSH-receptor subunit complexes by photoaffinity labeling. This experimental approach resulted in the formation of high molecular mass complexes (116, 172, and 320 kDa) as detected by autoradiography. After reversal of the BSOCOES cross-links, a second-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of these complexes demonstrated that two lower molecular mass complexes, 64 and 84 kDa, identified previously as specific components of the FSH receptor by photoaffinity labeling alone (Smith, R. A., Branca, A. A., and Reichert, L. E., Jr. (1985) J. Biol. Chem. 260, 14297-14303) were contained within the 116- and 172-kDa complexes. In addition, the 116-kDa complex was not found to be a component of the 172-kDa complex. Since the high molecular weight complexes have molecular weights large enough to contain additional unlabeled proteins, these data indicate the possibility that there are several distinct FSH receptor subunits. Furthermore, the observation of the 320-kDa band, taken together with the observed structural relationships, suggests that the FSH receptor may contain two each of three distinct subunits with approximate molecular weights of 32,000, 48,000, and 86,000, respectively.