Endoproteinase Lys-C, Mass Spec Grade, is a highly-purified serine protease that hydrolyzes specifically at the carboxyl side of lysines. Lys-C retains proteolytic activity under strong protein denaturing conditions such as 8M urea, which can be used to improve digestion of proteolytically resistant proteins. Lys-C, Mass Spec Grade, has optimal activity in the range of pH 7.0—9.0. This protease can be used alone or in combination with other proteases to produce protein digests for peptide mapping applications or protein identification by peptide mass fingerprinting or MS/MS spectral matching.