The influence of erythrocytes of different blood groups on the intestinal isoenzyme of alkaline phosphatase (EC 18.104.22.168) was investigated. Binding of the enzyme to erythrocyte membranes was demonstrated by sedimentation experiments, by enzyme electrophoresis, by gel chromatography and immunologically using a modified Coombs technique. We found that red cells of blood group A bind almost all intestinal alkaline phosphatase; erythrocytes of blood group B or O to a much lesser degree. This is in accordance with the fact that intestinal alkaline phosphatase is found more frequently in the serum of individuals of blood group O or B than in serum of persons of blood group A. We conclude that underlying the described blood group dependent differences in synthesis of intestinal alkaline phosphatase lies a mechanism involved in the regulation of the activity of intestinal alkaline phosphatase in human serum.