THE genetics of the Rhesus factor have turned out to be so complex and our understanding of it has advanced so rapidly that it is difficult for many to arrive at a clear picture of the situation now substantially established. The notation has been frequently changed, and we feel that only a notation which designates unambiguously the antibodies, the genes or gene-complexes, and the antigens with which these antibodies react can avoid widespread confusion. In Table 1 we set out such a notation suggested by Fisher1 which has been in use in this laboratory for about eighteen months. Six of the gene designations here adopted are due to Wiener2, but for the antibodies his notation seems arbitrary. While Cappell's names3 such as anti-C, anti-D, anti-E and anti-c are unambiguous, Wiener's do not seem satisfactory since, for example, the 85 per cent reacting serum is called anti-Rh0, whereas besides Rh0 it reacts with the genes Rhl, Rh2 and Rhz. In the designations here used, on the contrary, ? or anti-D indicates that the serum reacts with an elementary antigen D present equally in the gene complexes of Rh0, Rhl, Rh2 and RhZ as shown in Table 1. (For the remainder of this communication the h will be omitted from Rh.)