1. Twelve cases of immunization by pregnancy or blood transfusion in which the Rhesus antibody anti-E was found, to the exclusion of anti-D or any other Rh antibody are reported.
2. Two hundred and fifty Rh antibodies were discovered in this laboratory during 1949 in 37,972 antenatal cases not previously tested, i.e., 1 in 152. Seven of these were pure anti-E, i.e., 2.8 per cent. This frequency is considerably higher than any previously reported.
3. All of the anti-E sera tested possessed to some degree, and 5 of them to a marked degree, the property of distinguishing cells with a double from those with a single dose of the antigen E.
4. The presence, in cells of various genotypes, of tine antigen D in association with the antigen E reduced their agglutinability with the same anti-E serum in such a manner as to suggest that the agglutinability of these cells depends upon the relative quantities of the E and D anitigens present.
5. By means of parallel titrations with a genotyping anti-E serum, against cells of known and unknown genotypes, it was possible to distinguish E/E from E/e cells in the presence or absence of associated D and, as a corollary, to distinguish D/D from D/d cells in the presence of associated E/E or E/e.
6. These results fit the hypothesis that the specific Rhesus antigens C, D and E are produced from a limited amount of "basic raw material" by the activity of the three genes: that the genes act independently and in competition and that the amount of each antigen produced is proportional to the dosage and "competitive value" of the corresponding gene.