Naturally occurring anti‐M antibodies were found in the serum of a five‐month‐old Negro twin as revealed by a discrepancy between the cell blood group and the reverse blood group. The corresponding twin did not possess this antibody.There was no apparent cause for its presence, although it was of interest that the mother was type MN. The possibility is discussed that transfer of maternal red cells to the fetus occurred and that the M agglutinogen persisted in the infant following birth until the time that it was able to respond by forming anti‐M ant