A variety of factors capable of inhibiting the in vitro binding of FSH to its receptor have been identified in gonadal tissues from males and females. Interest in these factors has been stimulated because of their potential role as local modulators of gonadotropin action. Studies reported here were undertaken to determine if proteins having antigenic homologies with human FSH or an "FSH-like" protein isolated from porcine follicular fluid were present in human testicular tissue or seminal plasma. Polyclonal antibodies were generated against fractions of porcine follicular fluid containing FSH receptor binding inhibitory activity, FSH agonist activity in vitro, and a 58,000 Mr protein recognized by human FSH antiserum. Antiserum against this fraction of porcine follicular fluid and antiserum against human FSH were used to probe Western blots of proteins from human testis homogenates or seminal plasma. A 58,000 Mr protein was identified in both human testis extract and seminal plasma. This protein appears to be related antigenically to both human FSH and the 58,000 Mr "FSH-like" protein in porcine follicular fluid. It does not appear to be a metabolic degradatory product of human FSH since the protein is larger than FSH, does not dissociate into subunits under reducing conditions and is recognized by the antiserum to FSH-like protein that does not recognize human FSH. These data identify a 58,000 Mr protein in human testis and seminal plasma that may represent a local modulator of FSH action in the testis.
