We describe a genetic polymorphism of cytosol polypeptide with mol. wt. of 38,000 detected in phytohemagglutinin (PHA)-stimulated peripheral blood lymphocytes by two-dimensional gel electrophoresis. Three different electrophoretic phenotypes (type 1-1, 2-1, 2-2) of the polypeptide have been identified in a Japanese population. Family and population studies indicate that three phenotypes are determined by two common alleles at a single autosomal locus. Since the polypeptide is mainly present in cytosol of cells, we propose that the polypeptide be temporarily designated as cytosol polypeptide with mol.wt. of 38,000 (CP38) and that the gene for CP38 be designated as CP38. The gene frequencies of two common alleles (CP38(1) and CP38(2)) are 0.899 and 0.101, respectively, in a Japanese population. The data on gel filtration of cytosol proteins on a Sephadex G-100 column suggest that CP38 exists as a dimer in the cytosol. CP38 was observed in the wide range of different cells, including B-lymphoblastoid cells, adult skin fibroblasts, HeLa cells, and erythrocytes. In 11 out of 72 individuals, the phenotypes of CP38 were different from those of adenosine deaminase which is similar to CP38 in subunit size, cell distribution, and allele frequencies. These data indicate that CP38 is a new polymorphic polypeptide encoded by an autosomal locus.