ABSTRACT Immunological cross reactions among the three gonadotrophins, ovine interstitial cell-stimulating hormone (ICSH), pregnant mare serum gonadotrophin (PMSG) and human chorionic gonadotrophin (HCG) have been investigated. The earlier findings in this laboratory that rabbit antiserum to ICSH inhibited the biological activity of PMSG was confirmed. Antiserum to PMSG was found to inhibit ICSH. The biological activity of HCG in hypophysectomized female rats was not inhibited by antiserum to either ICSH or PMSG in these experiments, but other evidence shows that antiserum to ICSH is sufficiently high doses does inhibit the biological effects of HCG. Both PMSG and HCG showed precipitin reactions with antiserum to ICSH, the amounts of protein precipitated being much less than when ICSH was used. No precipitate was formed when ICSH or HCG was added to antiserum to PMSG. The antiserum to PMSG contained antibodies to the horse serum proteins that were present as contaminants of the PMSG. These were partly absorbed with horse serum without loss of the anti-PMSG activity.