A technique for the visualization of phosphoglycerate kinase on starch gel after electrophoresis is described. Three bands of activity were found in hemolysates prepared from normal red cells. When ATP, a substrate of the enzyme, was incorporated into the gel, only a single band was found. This suggested that ATP complexed with the enzyme and/or produced configurational changes. Incidentally, it was found that ATP markedly altered the electrophoretic mobility of hemoglobin. Red cells of 92 Caucasian males, 121 Caucasian females, 114 Negro males, 10 Negro females, 4 Oriental males, and 4 Oriental females were examined. No evidence of an electrophoretic polymorphism of this enzyme was found. Patterns of activity similar to those found in red cells were found in liver, heart, kidney, and skeletal muscle.