615 human sera taken from healthy donors, pregnant women and patients with various diseases were tested against human erythrocytes of blood group·which had been treated with trypsin. In healthy donors, 7 of 252 (2.8%) sera showed positive reaction, whereas in systemic lupus erythematosus 12 of 49 (24.5%) sera were positive. The agglutination occurred at an optimum temperature of 20 °C. Consistently negative results were observed in indirect antiglobulin tests. The agglutinin was a mercaptoethanol-sensitive macromolecule. It was apparently an immune antibody to an antigen which is present under the cell surface of all human erythrocytes.