Highly purified GAPD preparations from human erythrocytes and skeletal muscle have been used as immunogenes in rabbits. The antibodies produced readily precipitated their antigens and also inhibited their enzymatic activities. An immunochemical evaluation of the precipitability of both enzymes exhibited no immunochemical differences between them. Furthermore, the antibodies were tested against several tissue homogenates from man and revealed an identical precipitability. The identical cross-reactivity indicating a lack of antigenic differences support the absence of GAPD-isoenyzmes in man. The amount of GAPD in several organ extracts was estimated by the technique of single radial immunodiffusion. Furthermore it was demonstrated that the GAPD is bound to the membrane only under hypotonic conditions of hemolysis, while under approximated intracellular conditions of hemolysis the GAPD is not membrane bound.