Using synthetic peptides, we previously identified portions of the lutropin alpha-subunit that are in contact with the beta-subunit. In order to elucidate structure/function differences of the glycoprotein hormones, a similar study was conducted for follitropin. Peptides corresponding to the follitropin alpha-subunit were synthesized using standard solid-phase procedures. Purified peptides were incubated in the presence of alpha- and beta-subunits of follitropin, and subunit recombination was monitored using gel permeation chromatography and reverse-phase high-pressure liquid chromatography. Peptide alpha 33-58, corresponding to a highly conserved portion of alpha-subunit, completely inhibited subunit recombination for 24 h, and allowed only partial (61%-71%) recombination after 48 h. Peptide alpha 51-65 or alpha 61-78 inhibited subunit recombination partially at 24 h, but almost full (greater than 80%) recombination was observed by 48 h. Peptides corresponding to the rest of the alpha-subunit, alpha 1-15, alpha 11-27, alpha 22-39, and alpha 73-92, did not inhibit recombination of the alpha- and beta-subunits. The data suggest that alpha-subunits have similar residues in contact with regions of the beta-subunits of both lutropin and follitropin, specifically involving residues from continuous regions of the alpha-subunit (residues 45-75). The data suggest that this region contains multiple sites of contact with the beta-subunit.