Clonality of hematopoietic cells on a smale scale (nanogram amounts of DNA) can be detected by X-chromosome inactivation using the polymerase chain reaction (PCR). The human androgen-receptor gene (HUMARA) has a polymorphic short tandem repeat (STR), and has generally been used for clonality analysis since heterozygosity for the gene occurs in 90% of caucasian females. We examined heterozygosity of the STR on HUMARA in 110 Japanese females and found heterozygosity in 74 of 110 (67%). To examine for hematologic clonality in females with HUMARA homozygosity, we used a primer specific for a novel polymorphic STR site between DXS15 and DXS134 (DXS15-134) on Xq28. Heterozygosity for this site was found in 50 of 110 females (46%). Clonality of the hematopoietic cells was detected in 91 of 110 females (83%) using PCR of either the STR sites on HUMARA or DXS15-134. The X-inactivation patterns using PCR of DXS15-134 corresponded exactly with those obtained using PCR of HUMARA in 18 females who were heterozygous for both DXS15-134 and HUMARA. Using PCR of DXS15-134, we examined the clonality of bone marrow cells separated by flow cytometry in a patient with erythroleukemia (M6). Clonality was found not only in myeloid lineage cells but also in B lymphocytes. The clonality assay for DXS15-134 may be useful to assess for clonality of hematopoietic cells in the Japanese population, when combined with the HUMARA assay. Am.