In human seminal plasma a family of proteins that is immunologically related to the RSV-IV protein secreted under androgen control from the epithelium of the rat seminal vesicles was detected by a radioimmunoassay. Evidence for the origin of these antigens from human seminal vesicle is presented. Quantitative measurements of this family of proteins were performed in men with low levels of serum testosterone (idiopathic hypogonadotropic hypogonadism) and in individuals having serum testosterone in the normal range of values but carrying sex chromosome aberrations (Klinefelter's syndrome). In the first case we have found a marked decrease in the total amount of the RSV-IV-related proteins. An increase of about 40% in the total amount of these antigens was obtained in these subjects by gonadotropin treatment. A decreased amount of these proteins was also detected in the subjects affected by Klinefelter's syndrome. The possibility that some factor(s) under genetic control is involved, in addition to testosterone, in the regulation of this family of proteins is discussed.