Genetic polymorphism of the human parotid salivary acid phosphatase (s-AcP) in the Japanese population is described. The use of polyacrylamide gel isoelectric focusing electrophoresis with the pH range of 4.0-6.5 enabled us to discern three variant patterns controlled by two codominant alleles at the single autosomal locus. The two alleles were designated s-AcP:A and s-AcP:a, and the gene frequencies calculated from 183 Japanese subjects were s-AcP:A = 0.2268 +/- 0.022, s-AcP:a = 0.7732 +/- 0.022, respectively. The distribution of phenotypes fitted the Hardy-Weinberg equilibrium.