Crude enzyme solutions of prolidase were extracted from cultured human skin fibroblasts derived from control and prolidase-deficient sisters. Two forms of prolidases (prolidase-I and II) were partially purified by high performance liquid chromatography equipped with an ion exchange column. On gel filtration, the relative molecular weights of prolidase-I and II were estimated to be MW = 105,000 and 151,000, respectively. The substrate specificity of partially purified prolidase-I and II in control fibroblasts was estimated against Gly-Pro, Ala-Pro, Met-Pro. Each form of prolidase differed in its substrate specificity. In prolidase-deficient sisters, the elder with typical clinical manifestations and the younger with only slight clinical manifestations, the activity of prolidase-I was absent. However, the activity of prolidase-II was sufficiently present in both sisters. The substrate specificity of prolidase-II in the patients was similar to that of control. No difference in substrate specificity was found between these two patients.