ABSTRACT: Mouse hybrid cell clones secreting antigonadotropin releasing hormone monoclonal antibody were developed by fusion of SP2/O-Ag 1.4 myeloma cells with splenocytes of mouse immunized with gonadotropin releasing hormone (GnRH) tagged to tetanus toxoid. The product of hybrid cell clones obtained as ascites fluid from mouse peritoneal cavity had a titre of 106 (30–40% binding of 125I-GnRH) in radioimmunoassay. The antibody was IgG2a and Kappa. The association constant (Ka) of the product of hybrid cell clone P81662 for binding with GnRH was 1.2 × 109 L/mole. The monoclonal antibody (P81662) was highly specific for the native GnRH and devoid of reactivity with thyroid releasing hormone as tested in competitive radioimmunoassay. The recognition for GnRH agonists by monoclonal was 387-fold less with D-Ser (But)6 des Gly10 GnRH ethylamide and 608-fold less with Bz1-His6 GnRH. Monoclonal anti-GnRH antibody was competent to neutralize the in vivo bioactivity of the hormone as evident by the block of estrus cycle and termination of pregnancy in mice. Termination of pregnancy in animals receiving anti-GnRH monoclonal could be prevented by administration of progesterone.