We observed three cases in which anti‐M, undetectable in pretransfusion serum, was responsible for accelerated hemolysis of crossmatch‐ compatible red blood cells 5 to 15 days after transfusion. In each case, the direct antiglobulin test, which had been negative on pretransfusion testing, was weakly positive on the posttransfusion sample. Anti‐M was identified in both the serum and eluate from the posttransfusion sample in two cases. In the third, anti‐M could be identified only in the posttransfusion serum. Hemolysis was mild, and was not clinically suspected in any of these three patients until blood was requested for further transfusion. In one of the cases, the antibody was undetectable within a few weeks after the hemolytic episode. Destruction of transfused red blood cells by newly synthesized alloantibodies, particularly those of the Kidd system, is a familiar phenomenon to blood bankers. It is apparent from these studies that anti‐M also can behave in thi