We have compared the kinetic, immunological, and electrophoretic properties of human and canine erythrocyte pyruvate kinase. Both enzymes are allosteric and subject to positive and negative regulation. The allosteric properties of the canine enzyme are more pronounced than those of the human enzyme; however, the properties of both enzymes are consistent with a regulatory function in the glycolytic pathway of their respective erythrocytes. Antiserum against the human enzyme gives precipitin lines of partial identity between the human and canine enzymes on immunodiffusion. The anti-human serum inactivates the enzymatic activity of both enzymes, although it is more effective against the human enzyme than the canine. The two enzymes have slightly different mobilities on starch gel electrophoresis. While we have demonstrated differences between erythrocyte pyruvate kinase from dogs and that from humans, we conclude that the enzymes are sufficiently similar in properties and function to allow use of the dog as a model for human erythrocyte pyruvate kinase deficiency.