Serum samples treated with chondroitinase ABC and sialidase were investigated for the detection of inter-alpha-trypsin inhibitor (ITI) polymorphism. The improved phenotyping procedure has proved to be the most practical method for ITI phenotyping. The ITI allele frequencies were examined in 2 population samples from Japanese (n = 365) and Thais (n = 150). Three common alleles, ITI*1, ITI*2, and ITI*3 were identified in both populations, but the Thai population showed a higher frequency of ITI*1 and a lower frequency of ITI*3. Two new alleles were found, which were tentatively denoted ITI*Y and ITI*T. The ITI*T allele frequency in Thais was 0.047.