With the aim to produce antibodies which possibly would differentiate red cells of various human individuals, chimpanzees were injected with human blood. Whereas in this respect the results were not striking enough to warrant a detailed investigation, it may be worth while to put on record observations which have been made in the course of these experiments. Three chimpanzees, belonging to blood group A, were injected intramuscularly with washed human blood at intervals of seven to nine days, the animals receiving the following amounts in terms of whole blood: Chimpanzee 1 : 5, 5, 25, 25, 25 cc.; chimpanzee 2 : 10, 20, 15 cc.; chimpanzee 3: 10 cc. Titration of the sera are given in table 1. Tested in suitable dilutions, the chimpanzee immune sera differentiated sharply human and chimpanzee blood (table 2) with the exception of a particular sample to be mentioned (serum of chimpanzee 2).