A starch gel electrophoretic procedure is described that resolves peptidase S (PEPS) as well as the peptidases A, B, and C in man-rodent, rodent-rodent, and primate-rodent interspecific somatic cell hybrids. The interspecific PEPS cell hybrid phenotype can be resolved into a pattern which suggests that PEPS is composed of five or six identical subunits. Results are presented supporting assignment of the PEPS locus to chromosome 4 in man using man-mouse and man-Chinese hamster somatic cell hybrids. Human genes coding for peptidases A, B, C, and D were assigned to chromosome 18, 12, 1, and 19, respectively, confirming previous assignments. These somatic cell genetic data demonstrate the independent genetic control of the several human peptidases.