A new member of the serine protease inhibitor (serpin) superfamily with megakaryocyte maturation activity was purified, and its cDNA was cloned and characterized. The predicted amino acid sequence consisting of 380 residues was unique and was 38% identical to the serpin plasminogen activator inhibitor type 2 (PAI-2). The recombinant factor expressed in Chinese hamster ovary cells showed species-specific activity on the induction of megakaryocyte maturation in vitro. When injected into mice, the factor indeed elicited an increase in the number of platelets in plasma. The sequence alignment indicated that the factor possessed a lysine residue at the P1 position, suggesting that it might function as an inhibitor of Lys-specific proteases. Although we could not show any inhibitory activities toward several known Lys-specific proteases, we detected the activity toward protease activity present in the culture supernatant of COLO 201 cells. These results suggested that the protein might influence the maturation of megakaryocytes via action as a serpin.