The nuclear protein statin, detectable with specific monoclonal antibodies, is found mostly in nonproliferating cells (Wang, E. (1985) J. Cell Biol. 100, 545-551). In the rat liver a 57-kDa protein designated as rat liver protein 57 (RLp57) was recently identified to carry the epitope for the anti-statin-specific monoclonal antibody, S-44 (Sester, U., Moutsatsos, I. K., and Wang, E. (1989) Exp. Cell Res. 182, 550-558). To characterize further the RLp57 protein, in the present study a polyclonal antibody was raised to the RLp57 protein eluted from polyacrylamide gel. Similar to the anti-statin monoclonal antibody, this polyclonal antibody recognizes a nuclear antigen in nonproliferating fibroblasts and reacts with a 57-kDa protein in rat liver and nonproliferating cells strongly suggesting that RLp57 is a statin protein from rat liver. Two isoforms of RLp57 (isoelectric points between 6.5 and 7.0) were detected after two-dimensional gel electrophoresis and immunoblotting. RLp57 was purified using multiple chromatographic steps, including ion-exchange and affinity chromatography followed by chromatofocusing. These results show that RLp57, a statin protein found in liver, has two isoelectric variants and can be purified to apparent homogeneity by sequential steps of chromatographic procedures.