Summary and Conclusions: The reactions obtained with a variety of anti-M and anti-N fluids, confirm the original observation of Landsteiner and Levine that the blood of all chimpanzees contains agglutinogens similar to but not identical with the human agglutinogens M and N. Moreover, it could be shown that M agglutinogens are also present in the blood of gibbons, orang-utans and certain species of lower monkeys. The degree of resemblance between the M agglutinogens in monkey or ape blood and human blood shows a certain degree of correlation with the position of the monkey or ape in the evolutionary scale. From the behavior of different anti-M sera in tests on monkey blood, the existence of a number of qualitatively different M agglutinins may be assumed, yet all the sera behave alike in tests on human blood. Which M agglutinins will be formed when rabbits are immunized with human M blood evidently depends upon constitutional attributes of the rabbit.