Rabbits and turkeys were immunized against commercial preparations of pregnant mare serum gonadotrophin (PMSG). Ouchterlony double diffusion and haemagglutination-titration tests showed that the rabbit antisera contained non-specific antibodies against horse serum proteins, whereas the turkey antisera did not. Both types of antisera neutralized the effects of PMSG in vivo. One specific turkey anti-PMSG serum was used unabsorbed to establish a sensitive and accurate haemagglutination—inhibition assay for PMSG in equine serum and in saline. Pyruvic aldehyde-treated sheep red blood cells sensitized with PMSG were used in perspex haemagglutination trays. The reliability criteria of the assay is discussed. The smallest quantity of PMSG detectable in saline was 0·3 i.u./ml. The turkey anti-PMSG serum showed a strong immunological cross-reaction with preparations of equine pituitary follicle-stimulating hormone (FSH) and equine pituitary luteinizing hormone (LH), a faint reaction with human pituitary gonadotrophin and no reaction against human chorionic gonadotrophin, ovine FSH, ovine LH, bovine LH or porcine FSH.