A number of patients with a male phenotype and a female (46XX) karyotype were described. Although there is little or no evidence for the presence of a Y chromosome in their cells, these individuals resemble patients with Klinefelter syndrome (47XXY). Using a new serological assay for the presence of H-Y antigen, a cell surface component associated with the Y chromosome, the presence of Y-chromosomal genes in a 46 yr old man with an XX karyotype is demonstrated. In addition, using standard cytological techniques, a minor population of XXY cells as well as cells bearing an abnormal chromosome 17 among the blood leukocytes of this individual was located.