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The capacity possessed by some rabbit immune sera produced with blood of Rhesus monkeys, of reacting with human bloods that contain the agglutinogen M has been reported previously.', Subsequently it has been found that another individual property of human blood (which may be designated as Rh) can be detected by certain of these sera.Upon exhaustion of such a serum with selected bloods, for instance OM, the absorbed serum still agglutinated the majority (39 out of 45) of other human bloods, independently of the group or the M,N type ; moreover, reactions took place with bloods lacking the property P. An example of the reactions is given in Table I. TABLE I. Bloods (all group 0) r-Type M TypeN TypeM,N 1 2 3 4 5 6 7 8 9 1 0 ---Absorbed immune serum + + + o o + + + o + Technic: Immune serum for Rheaus blood diluted l:lO, absorbed with half volume of sediment of blood 4. One drop each of absorbed serum, cell-suspension (2%) and saline used. Positive agglutination designated by + sign.The results are of some interest in that they suggest a way of finding individual properties in human blood, namely, with the aid of immune sera against the blood of animals. As an analogy may be cited the demonstration of differences in sheep erythrocytes with immune sera for human A The reactions described, although of moderate intensity only, were obtained with immune sera produced at different times. Whether these observations may possibly lead to a method suitable for routine work is still under investigation.Readings after 2 hours at-room temperature.

From some observations made with immune sera, and particularly from the evidence provided by tests with occasionally occurring normal and post-transfusion human sera containing irregular agglutinins (cf. reviews in 1, 2) one can conclude that there exist individual properties of human blood other than those which are demonstrable by readily available reagents such as A1, A2, B, M, N. Doubtless numerous attempts have been made to discover additional agglutinogens by the familiar technique used for the demonstration of the factors M and N (3), that is, with immune sera prepared by the injection of human blood into rabbits, but only few results were obtained (e.g. 4, 5), and these were not followed up because it was difficult to produce the immune sera again. Other ways of approaching the problem were therefore desirable and it was thought that new results might be obtained by immunizing with animal instead of human blood, considering that the blood of some animals contains antigens related to agglutinogens present in individual human bloods, for instance the Forssman substance related to A in sheep cells. A result that favored this plan was the observation that certain anti-rhesus immune sera contain agglutinins specific for the human agglutinogen M (6).Pursuing this idea, by immunizing rabbits with rhesus blood an immune serum was obtained with which an agglutinable factor different from A, B, M, N, or P was detected (7), and this new factor was designated as Rh to indicate that rhesus blood had been used for the production of the serum. The property was then found to be present in the blood of about 85 per cent of white individuals examined (7,8).Evidence that the property Rh is of clinical importance was obtained when one of the writers came into possession of blood samples from patients who had shown hemolytic reactions, one with fatal outcome, after receiving repeated

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