During studies of agglutination of trypsinized human red blood cells (RBC) by anti-Rh sera, it was noticed that some human sera without Rh antibodies agglutinated these erythrocytes. Of 933 normal and pathological sera subsequently screened, 116 produced such agglutination. The agglutinins were of IgM nature and were directed against an antigen exposed or created by tryptic digestion of RBC. They reacted equally well with autologous as with allogeneic trypsinized RBC and were different from T agglutinins and from Paul-Bunnell antibodies. The mode of formation of these antibodies remains unknown.