In a patient with hypogonadotropic hypogonadism treated with luteinizing hormone releasing hormone (LHRH), secondary failure of subjective and hormone responses occurred at the time of appearance of binding of 125I-LHRH by the patient''s serum. On electrophoresis of the patient''s serum with 125I-LHRH, label was found only in the .gamma.-globulin region. 125I-LHRH added to the patient''s serum was precipitated by sheep antihuman immunoglobulin[Ig]G (anti-IgG) but not by sheep anti-human IgM (anti-IgM). Competitive displacement of 15tI-LHRH by unlabeled LHRH was demonstrated, while TSH [thyroid stimulating hormone] releasing hormone (TRH), somatostatin and rat pituitary hormones showed no displacement when tested at concentrations 5 .times. 106 greater than that of LHRH. Studies using 14 different analogs of LHRH revealed that those with changes at the carboxy terminus showed binding similar to LHRH. IgG antibody to LHRH was produced in this patient by repeated administration of synthetic LHRH. Antibody specificity is directed toward the N terminus region.