The nucleotides (nt) at positions 467 and 646 of the ABO blood group system were analyzed in a Japanese population by means of the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) methods. Two types at nt467, tentatively designated 'Pro' and 'Leu', were found in the common A (= A1) alleles, and two types at nt646 named 'T' and 'A' were found in O alleles. The types at nt467 of B and O alleles were Pro and those at nt646 of A and B alleles were T. Therefore, A alleles were divided into A(Leu) and A(Pro) suballeles and O alleles were divided into O(T) and O(A). The allele frequencies in the present survey were calculated as ABO*A(Pro) = 0.0712, ABO*A(Leu) = 0.2163, ABO*B = 0.1779, ABO*O(T) = 0.2731 and ABO*O(A) = 0.2615. No O2 (or O(3)) allele was observed in the population samples. At least five alleles with polymorphic frequencies and 15 genotypes are present in the Japanese sample.