ABH carbohydrate antigens are cell surface carbohydrates which occur in three allelic forms, namely A, B and O blood groups. It is unknown how the ABO blood group is expressed in hemopoietic stem cells. In an attempt to verify the ABO mRNA expression in hemopoietic precursor cells, mRNAs were isolated from human chronic myeloid leukemia (CML) cell lines which are believed to be at the most immature level of hemopoietic differentiation among hemopoietic malignancies. In particular, K-562 and KOPM-28 cells were used with the reverse transcription-polymerase chain reaction (RT-PCR) technique for amplifying ABO gene transcripts. The amplified ABO cDNAs from two cell lines were characterized by the digestion of Kpn-I restriction enzyme. The blood types were determined by polymerase chain reaction of the specific allele (PASA) method. Both of the human chronic myeloid leukemia cell lines expressed ABO mRNA. The quantity of ABO mRNA in the K-562 cell line is significantly higher than that of the KOPM-28 cell line. The ABO blood type of these two cell lines was type O. Because the CML cell lines are presumed to be at the immature stem cell level of hematopoietic cell differentiation and because it is believed that the cultured cell lines from hematologic malignancy reflect the characteristics of normal corresponding hemopoietic cells, the hemopoietic stem cells should express mRNA of the ABO blood group.