AbstractSpecific antibodies against the human haptoglobin (HP) α chain were raised and used for immunoblotting after isoelectric focusing to determine Hp α subtypes in a reproducible and simplified manner. By eliminating the staining of the HP β chain, HP α subtypes were visualized more precisely and simply than by previously reported methods. Subtypes of a total of 1211 sera, obtained from two different populations in Japan, were examined by the new method. Four HP α subtypes (HP 1S–1S, 2FS–1S, 2FS–2FS and 2FS–2SS) and five subtypes combined with a new variant (HP 2FS–1V1, 2V1–1S, 2FS–2V1, 2FS–2V2 and 2FS–2V3) were observed. HP 1V1 belonged to HP α1, and HP 2V1, 2V2 and 2V3 belonged to HP α2, and their alleles were designated HP A*1V1, HP A*2V1, HP A*2V2 and HP A*2V3, respectively. The allele frequencies were calculated to be as follows: HP A*1S, 0.2597; HP A*1V1, 0.0004; HP A*2FS, 0.7333; HP A*2SS, 0.0008; HP A*2V1, 0.0045; HP A*2V2, 0.0008; and HP A*2V3, 0.0004. The allele frequency of HP A*2SS, which is common in the European population, is less frequent than HP A*2V1 in the Japanese population.