BACKGROUND: The knowledge about the nucleotides sequence of 9th chromosome that regulates the phenotype of ABO blood group has made the ABO genotyping possible. ABO genotyping can play important role in paternity and forensic investigation. Blood typing of the B subgroup is possible by genotyping the B gene because the B subgroup includes cis AB which has partially the same genes of normal B blood group. We evaluated the usefulness of ABO genotyping using the polymerase chain reaction (PCR) method as alternative test for blood typing in the three cases of Bx phenotype. METHODS: According to the general blood typing method which includes cell typing, serum typing, adsorption elution test and hemagglutination inhibition test of saliva, we identified the three Bx phenotype by weak agglutination with anti-A, anti-B serum and the presence of H substance in saliva. Genotyping by PCR method was performed by using three primers specific for O allele, B allele and A/O allele by Fukumori et al. RESULTS: Of the three cases of Bx phenotypes for genotyping, all three cases were typed as BO genotype showing full agreement with phenotyping. CONCLUSION: ABO genotyping will be useful as a supplemental and confirmatory test to solve the problem of blood group typing caused by mismatch between cell and serum typings, especially in B subgroup and when RBCs and saliva are not available for serological phenotyping.