A genetic polymorphism of human urine deoxyribonuclease I (DNase I) has been detected by the technique of polyacrylamide gel isoelectric focusing (IEF-PAGE) followed by immunoblotting with anti-DNase I antibody. Family studies showed that the three common phenotypes - DNASE1 1, 1-2, and 2 - and the other four rare phenotypes - DNASE1 1-3, 2-3, 2-4, and 3-4 - represent homozygosity or heterozygosity for four autosomal codominant alleles, DNASE1*1, *2, *3, and *4. The frequencies of the DNASE1*1, DNASE1*2, DNASE1*3, and DNASE1*4 alleles in a studied Japanese population were 0.5453, 0.4396, 0.0117, and 0.0034, respectively.