The distribution of a small Zn(2+)-binding protein (11.5 kDa ZnBP), which we have shown to be identical with parathymosin, was studied in various rat tissues by immunocytochemistry, immunoblot analysis and quantified by enzyme-linked immunosorbent assay (ELISA), using monospecific polyclonal antibodies. The content in liver was 105 micrograms/g wet weight. Similar amounts were found in brain, adrenal gland and smooth muscle, whereas in testis, spleen, lung, and kidney about half the amount was detected. Very low levels were found in skeletal muscle (2 micrograms/g) and adipose tissue, while erythrocytes did not contain measurable amounts. The specificity of the antibodies was established by immunoblotting. Purified 11.5 kDa ZnBP as well as 11.5 kDa ZnBP detected in crude soluble fractions from various tissues appeared always as a doublet of protein bands of about equal intensity, indicative for two isoforms of the 11.5 kDa ZnBP. By immunocytochemistry, in brain, high concentrations of 11.5 kDa ZnBP were found in the deep cerebellar nuclei, in soma and dendrites of Purkinje cells, and in the large neurons of the pons/medulla. In most cell types reacting with the antibody, exclusively the cytoplasm was stained. In contrast, in duodenal and jejunal crypt cells immunostaining was restricted to the nuclei, whereas the more mature cells at the top of the villi contained most of the antigen in the cytosol. Immunostaining of the nuclei was also observed in pancreatic duct cells, whereas in the duct cells of the parotid gland immunostaining was detected exclusively in the cytoplasm. In both tissues immunostaining of the acinar cells was negative.