The HY antigen is closely associated with the presence of testicular tissue which is thought to be induced during embryogenesis by the interaction of HY and its receptor. In coded samples, we examined the HY antigen in peripheral lymphocytes from a patient with male pseudohermaphroditism and dysgenetic testes whose karyotype in lymphocytes and gonadal fibroblasts was 46 XY. Results are as follows (mean + SD): The patient's lymphocytes exhibited an HY phenotype intermediate between that of the normal male and normal female and between that of the patient's mother and father. These results indicate a correlation between HY phenotype and abnormal testicular development and suggests the possibility that a partial deficiency of HY may have been responsible for the testicular dysgenesis in this patient.