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At the beginning of the twentieth century, Landsteiner (1, 2) reported the presence of normal isohemagglutinins in human serums and their corresponding agglutinogens in the red cells. On the basis of these observations, normal human blood is classified into 4 groups, i.e., Groups 0, A, B, and AB. Continued research has served to confirm Landsteiner's observations and postulates. Landsteiner found further that an isohemagglutinin and its homologous agglutinogen could not exist together in the same blood, and that their relationships are reciprocal; i.e., the presence of one indicates the absence of the other.With the advent of whole blood transfusions as an important therapeutic procedure, it became necessary to determine the blood groups of both donors and recipients prior to transfusion. A number of methods have been described for assigning an unknown blood to one of the 4 blood groups. In practice, they involve the collection and preservation of 2 grouping serums, containing, respectively, avid Anti-A and Anti-B agglutinins, preferably in high titer. The isohemagglutinin serums are mixed with the cells of the unknown blood under appropiate conditions which will give prompt true agglutination

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