The nuclear protein statin, found uniquely in nonproliferating cells, has been previously identified by mouse monoclonal antibodies designated as S-30 and S-44. We report here the screening of various rat tissues for proteins cross-reacting with these antibodies. As revealed by immunoblotting, two polypeptides (mol wt 80 and 57 kDa) and a group of lower-molecular-weight proteins migrating between 34 and 38 kDa were found to react with the anti-statin antibodies. The most prominent immunoreactivity was observed with a 57-kDa protein present in rat liver. Upon further fractionation of the liver protein extract with ammonium sulfate [(NH4)2SO4] the 57-kDa protein, designated as rat liver protein 57 or RLp57, was detected independently with both anti-statin antibodies in the 30 to 60% (NH4)2SO4 fraction. In order to determine whether rat liver protein 57 is indeed specifically recognized by anti-statin antibody S-44, we used RLp57 transferred onto nitrocellulose paper as a specific substrate for the adsorption of the S-44 antibody from ascites fluid. As shown by indirect immunofluorescence microscopy the solution remaining after adsorption failed to stain human fibroblasts. The adsorbed immunoglobulin, however, upon elution revealed statin-specific nuclear staining activity on senescent fibroblasts. These findings suggest that rat liver protein 57 and the human fibroblast statin share similar antigenic determinants recognized by the statin-specific S-44 antibody. Our results indicate furthermore that the statin previously identified in fibroblasts may represent one member of a group of several antigenically related proteins detectable with specific anti-statin antibodies.