The crystal structure of a βThr26Ala mutant of human follicle-stimulating hormone (hFSH) has been determined to 3.0 Å resolution. The hFSH mutant was expressed in baculovirus-infected Hi5 insect cells and purified by affinity chromatography, using aβ hFSH-specific monoclonal antibody. The βThr26Ala mutation results in elimination of the βAsn24 glycosylation site, yielding protein more suitable for crystallization without affecting the receptor binding and signal transduction activity of the glycohormone. The crystal structure has two independent hFSH molecules in the asymmetric unit and a solvent content of about 80%. The α- andβ subunits of hFSH have similar folds, consisting of central cystine-knot motifs from which three β-hairpins extend. The two subunits associate very tightly in a head-to-tail arrangement, forming an elongated, slightly curved structure, similar to that of human chorionic gonadotropin (hCG). The hFSH heterodimers differ only in the conformations of the amino and carboxy termini and the second loop of the β-subunit (L2β). Detailed comparison of the structures of hFSH and hCG reveals several differences in the β-subunits that may be important with respect to receptor binding specificity or signal transduction. These differences include conformational changes and/or differential distributions of polar or charged residues in loops L3β (hFSH residues 62–73), the cystine noose, or determinant loop (residues 87–94), and the carboxy-terminal loop (residues 94–104). An additional interesting feature of the hFSH structure is an extensive hydrophobic patch in the area formed by loops αL1, αL3, and βL2. Glycosylation at αAsn52 is well known to be required for full signal transduction activity and heterodimer stability. The structure reveals an intersubunit hydrogen bonding interaction between this carbohydrate and βTyr58, an indication of a mechanism by which the carbohydrate may stabilize the heterodimer.