H-Y antigen, the proposed inducer of testicular organogenesis, was determined serologically in 3 patients with male pseudohermaphroditism due to Leydig cell hypoplasia, a pathological model with lack of Leydig cell differentiation but normal seminiferous tubule embryogenesis. One patient was the offspring of consanguineous parents and 2 siblings presented as women with a lack of breast development and primary amenorrhea. Gonads were palpable in the inguinal canal, except for the right intra-abdominal testis in 1 patient. Two patients had female external genitalia and 1 had partial labial fusion. Karyotypes were 46XY. Gonadotropin levels were elevated, and testosterone was low and failed to increase after stimulation with human chorionic gonadotropin. Testosterone precursors were not elevated. Testicular histology showed absence of mature Leydig cells but relatively preserved seminiferous tubules. Family history was consistent for autosomal recessive inheritance. H-Y antigen expression measured by an enzyme-linked immunosorbent assay was normal, indicating that lack of other inductive factors for Leydig cell differentiation are responsible for Leydig cell hypoplasia.