Summary and Discussion: The results of experiments with the technic of typing for M and N have been presented and discussed. The most important requirement for accurate results is the proper potent reagents. For the preparation of potent sera, prolonged and intensive immunization is necessary, and when preparing anti-M sera the desired effect is often obtained by giving the rabbits a long rest period (one or more months) followed by an additional brief course of injections. The preparation of the anti-N testing fluid from the immune serum offers more difficulty than the preparation of anti-M fluids, because of the greater tendency to non-specific absorption with the former. However, there is usually no difficulty in preparing fluids with a titer of at least 10 and 20, and with reagents of such potency accurate results are possible. The majority of bloods of the heterozygous type are less sensitive to agglutination than bloods of the corresponding homozygous types, so that it is important to include an adequate number of control bloods of each of the three types M, N, and MN in every experiment. Other factors which must be taken into account are the possible presence of interfering agglutinins, which can be removed by suitable absorption experiments, and the effect of variations of the concentration of the suspension, the age of the blood, and temperature on the reactions. In medicolegal cases, absolute reliance should not be placed on any single set of reagents, and, of course, for such work adequate experience is of prime importance.