1. The theory of six allelic genes is reviewed, using the new improved nomenclature, and data are summarized regarding the distributions of the eight Rh types among white and negro individuals in New York City.
2. Results are presented of tests for property Hr in a series of 239 white individuals and 49 negroes. Statistical analysis of these data yields results supporting Race and Taylor's hypothesis that anti-Hr sera react with the blood properties determined by the genes Rh2, Rh'' Rho, and rh, but not with the factors determined by genes Rh1 and Rh'.
3. The practical importance of the Hr factor is discussed.