125I-Human growth hormone was cross-linked with high efficiency to lactogen receptors in membranes and in Triton X-100-solubilized preparations from luteinized rat ovaries using the bifunctional reagent disuccinimidyl suberate. Cross-linked samples were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions followed by autoradiography of dried gels. Analysis of cross-linked hormone-receptor complexes in membranes revealed the presence of a major radioactive band at Mr = 60,000. Cross-linking of detergent-solubilized complexes resulted in the appearance of two major radioactive bands of Mr = 60,000 and Mr = 100,000. Excess unlabeled lactogenic hormones inhibited the labeling of the major bands while non-lactogenic hormones had no effect. The same major radioactive bands were observed when the cross-linked samples were analyzed under nonreducing conditions. However, in solubilized preparations, the ratio between the intensities of the lower and the higher Mr bands was markedly lower under nonreducing than under reducing conditions. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis studies (first dimension, nonreducing; second dimension, reducing) demonstrated that a species with Mr = 60,000 can be released from the Mr = 100,000 species upon cleavage of disulfide bonds. Since the Mr of 125I-human growth hormone is 22,000, these results suggest that the ovarian lactogen receptors are molecules with an approximate Mr of 80,000, containing a subunit with a Mr of approximately 40,000 bearing the recognition site for the hormone. Some of these subunits appear to be linked to the rest of the molecule by disulfide bonds.