Human placental receptors for insulin and somatomedin C (Sm-C) were affinity labeled with [125I]insulin and [125I]Sm-C by using the bifunctional cross-linking agent disuccinimidyl suberate. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that both labeled hormones were specifically cross-linked to three protein species with apparent molecular weights of 240 000, 310 000, and 330 000. Following disulfide bond reduction, subunits of approximately 140 000 daltons were evident. Partial reduction of disulfide bonds yielded intermediate-sized species with apparent molecular weights of 180 000, suggesting the existence of an additional, smaller subunit attached to the 140 000-dalton subunit. Limited proteolysis of the hormone-receptor complexes with chymotrypsin, trypsin, and Staphylococcus aureus V-8 protease gave similar but not identical results for each labeled receptor. The distinction between the two receptors was further documented by inhibition of affinity labeling with graded amounts of the native hormones. These data demonstrate a substantial structural similarity between the human Sm-C and insulin receptors paralleling the homology of the native hormones and their actions.