Monoclonal antibodies were prepared against human follitropin by fusion of the myeloma cell line P3-X63-Ag8-653 with spleen cells of mice immunized with either human follitropin or follitropin bound to an anti-beta hFSH monoclonal antibody. The latter immunization procedure permits shielding of the immunodominant specific site and allows the production of numerous specific monoclonal antibodies to human follitropin which are complementary to the monoclonal antibody used in the immunogen. In this investigation two specific monoclonal antibodies were used in a two site immunoradiometric assay which was highly specific, rapid, with one incubation step and demonstrated a sensitivity level of 0.1 mIU/ml. It was possible to differentiate between prepubertal and adult levels of follitropin and to recognize individuals with hyposecretory states.