Endoproteinase Arg-C is a serine endopeptidase capable of hydrolyzing peptide bonds at the carboxyl side of arginyl residues, and exhibits both esterase and amidase activities. It can be isolated from Clostridium histolyticum. Endoproteinase Arg-C is of mass spectrometry grade and is applicable for peptide mapping, sequence analysis, and cell isolation.