The erythrocyte membrane protein pattern of patients with megaloblastic anemia was determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. In severe megaloblastic anemia, secondary either to folic acid or vitamin B12 deficiency, the erythrocyte membrane protein pattern was grossly abnormal, lacking bands 1, 2 (spectrin), and 3 and having several diffuse, faster migrating bands. After adequate vitamin replacement therapy, the erythrocyte membrane protein pattern returned to normal. In mild megaloblastic anemia, secondary either to folic acid of vitamin B12 deficiency, and in severe iron deficiency anemia, the erythrocyte membrane protein pattern was normal. Erythrocyte membrane protein pattern of normal membranes did not change after mixing with abnormal membranes before polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Protease activity extracted from membranes of megalocytes was not different from normal. These findings indicate that the erythrocyte membrane protein pattern is abnormal in severe megaloblastic anemia and that this abnormality is not secondary to increased activity of the endogenous erythrocyte membrane proteinase.