Glucose-6-phosphate dehydrogenase from rats and from humans was partially purified and was inactivated (dissociated) by removing triphosphopyridine nucleotide by repeated washing and dialysis, and then it was reactivated by addition of triphosphopyridine nucleotide and incubation at 25 degrees C. When enzyme from each species was mixed with enzyme from the other during or after inactivation, a new electrophoretic band located between the fast-moving rat dehydrogenase and the slow-moving type B human enzyme appeared. This band is interpreted as representing a hybrid composed of polypeptide chains from each enzyme.