We constructed a cDNA library using mRNA isolated from liver 48 hr after hepatectomy (HX) and screened it by differential hybridization using cDNA from normal and regenerating rat liver. We isolated one clone termed regeneration-associated serpin-1 (rasp-1) that was expressed in normal liver but was upregulated approximately 3-4 fold by 48 hr after HX. DNA sequence analysis of rasp-1 indicated that it encoded a novel 436 amino acid secreted protein. Moderate homology was found with several members of the serpin family of serine-protease inhibitors. The 1.7 kb raps-1 mRNA was highly expressed in liver but not in brain, heart, kidney, lung, testis or spleen. We also found the RASP-1 protein in normal and HX rat plasma using a polyclonal antibody generated against a deduced peptide of rasp-1. Rasp-1 may encode a novel serine-protease inhibitor associated with liver regeneration.