Antisera were produced in rabbits against highly purified preparations of human LH (2000 or 10,000 i.u./mg), human FSH (5500 i.u./mg), and human TSH (7-5 i.u./mg). Most rabbits produced antisera of high titre and high avidity. Cross-reactions were minimal between human TSH and human chorionic gonadotrophin (HCG) and between human FSH and HCG but marked between human LH and HCG. TSH and FSH also showed a constant but relatively weak cross-reaction. LH cross-reacted with FSH to a higher degree than did HCG. The avidity of the antisera was high. It was concluded that much of the lack of specificity recorded for glycoprotein antisera are effects of impure immunogens. Some of the true cross-reactions are probably explained by shared antigenic determinants of the beta-subunits. Unadsorbed antisera could be used for assay of FSH and TSH in plasma from pregnant women.