A detailed procedure is described for a rapid detection of phosphoglucomutase-2(=phosphopentomutase;PGM-2) on Cellogel following electrophoresis of extracts of human red blood cells and other tissues, including cultured fibroblasts and various types of primate-rodent somatic hybrid cells. The present study indicated that there is only one locus for phosphopentomutase in man. The data from a selected panel of 20 independent clones of man-mouse somatic cell hybrids, investigated for the presence of human chromosomes and for the presence or absence of human PGM-2 favored the assignment of the human PGM-2 locus to chromosome 4.