Studies were performed to try to determine if gonadotropins are altered during their interaction with tissue receptors. Immunologic, electrophoretic and binding properties of lactoperoxidase labeled [125I]HLH [human luteinizing hormone] and [125I]HCG [human chorionic gonadotropin] were examined before and after elution from mouse luteoma and human corpora lutea receptor preparations. The anti-HCG used in these studies at a 1:10,000 dilution precipitated 92% of a freshly iodinated [125I]HCG preparation. Receptor eluted [125I]HCG, derived from the same batch of labeled ligand, was virtually quantitatively precipitated by the same dilution of anti-HCG. [125I]HCG eluted from the human corpus luteum was electrophoretically more homogenous when compared to its heterogenous parent labeled preparation and migrated to a position similar to that of native HCG. In Ouchterlony double diffusion experiments against anti-HCG antiserum, corpus luteum eluted [125I]HCG and [125I]HLH showed immunologic identity with each other and with native HCG and HLH. Receptor eluted [125I]HCG from the mouse luteoma, following in vivo administration via tail vein injection or after incubation in vitro with labeled hormones, was immunologically indistinguishable from native HCG. The electrophoretic mobility of HCG was retarded when HCG was added to extracts of mouse luteoma, liver and kidney. Eluates of mouse luteoma, applied to Bio-Gel columns previously equilibrated with [125I]HCG, showed the ability to concentrate [125I]HCG in the high molecular weight column fractions. Similar results were obtained with columns equilibrated with [125I]TSH [thyrotropin] and [125I]HGH [human growth hormone]. [125I]HCG eluted from the mouse luteoma was able to bind to fresh luteoma homogenate but, in contrast to an earlier report with [125I]HCG eluted from rat testis, no enhancement of binding of the eluted [125I]HCG was observed with fresh tissue. These results could be explained by the extraction of non-dialyzable intracellular component during the [125I]HCG elution procedure from the luteoma homogenate which combines with HCG to lower its binding and alter its electrophoretic mobility. This component could be extracted from other mouse tissues and combines with other labeled peptide hormones. Gonadotropins eluted from mouse luteoma and human corpus luteum are not altered by their interaction with tissue receptors.