The bloods of twenty-one anthropoid apes have been examined serologically. It was found possible to assign each to one of the four human blood groups. The isoagglutinogens of the anthropoids were found to be identical with those of human blood. These very same factors could not be demonstrated in the blood of the lower monkeys. In the blood of the fourteen chimpanzees examined only the isoagglutinogen A has been found, whereas both A and B were present in the blood of the six orangs. The significance of these findings for the knowledge of the human blood groups is discussed.