Unexpectedly high LH and FSH serum levels, measured by conventional RIA methods employing a rabbit antibody, were observed in eight women 18 to 34 years of age who came to our observation for acne and/or hirsutism and with no sign of premature ovarian failure. Reinvestigation of patient histories revealed that all of them had previously received multiple injections of a multimicrobial vaccine containing rabbit proteins. Luteinizing hormone and FSH were remeasured by IRMA and ELISA. These techniques revealed that serum levels of both gonadotropins were within the normal range in all subjects. We believe that falsely elevated gonadotropin levels were induced by the presence of heterophilic antibodies against rabbit serum interfering in the conventional RIA.