SUMMARYThe complex serologic behavior of the Rh-Hr agglutinogens has been underlined by the recent discovery that, with very rare exceptions, associated with blood factorRh0of Rh-positive blood, there are numerous other blood factors which we designatedRhA, RhB, RhC. Rare Rh-positive individuals exist whose bloods have blood factorRh0but lack one or more of the other components. Such individuals can and have become sensitized to the missing blood factor. For example, in the case of an Rh-positive individual lacking blood factorRhA, anti-RhAmay be produced. So, too, whenRhBandRhC, are lacking, anti-RhBor anti-RhCmay be produced. In fact, all 3 have been identified. The resulting anti-RhA, anti-RhBand anti-RhCserums are indistinguishable from “standard” anti-Rh0serum in parallel tests on a random series of blood specimens, unless the series happens to include one of the rare Rh-positive bloods lacking blood factorRhA(typeRh0aor typeRh1a, etc.).In the present paper, anti-RhAserum from a sensitized typeRh1amother whose child had erythroblastosis was used for studies on the distribution and heredity of theRhAblood factor. In addition, attention is called to the fact that Unger and Wiener have identified anti-RhBand anti-RhCalthough they are reporting at this time their studies with anti-RhA. A total of 2012 blood specimens fromRh0-positive individuals were tested; 951 from Caucasoids and 918 from Negroids. In tests on blood from Caucasoids, a “standard”Rh0blood factor was invariably associated with a “standard”RhAblood factor. In no instance where the reactions with anti-Rh0serums were typical was theRhAfactor absent or a variant. However, if theRh0factor was a variant, 3 possibilities with regard to factorRhAwere identified. Either factorRhAwas “standard”, or factorRhAwas a variant, or factorRhAwas absent. This last possibility rarely occurs in Caucasoids since in our series only 1 Caucasoid blood or 0.1 percent lackedRhAblood factor and in that case theRh0factor was a variant.Among the 918 Rh-positive blood specimens from Negroids examined the situation was found to be somewhat different. While a “standard”Rh0, was almost always associated with a “standard”RhA, in 0.9 percent theRhAfactor was absent. Among bloods with aRh0variant blood factor, just as with Caucasoids, all 3 possibilities were identified, namely bloods with “standard”RhA, withRhAvariant, and also bloods with blood factorRhAabsent. The incidence of Rh-positive bloods lacking factorRhAwas considerably higher among Negroids than among Caucasoids, namely, 1.6 percent in Negroids, as compared with only 0.1 percent in Caucasoids.One interesting family was studied. The father's blood had the “standard”Rh0blood factor, but lacked theRhAcomponent (type Rh0a). The mother's blood had theRh0variant blood factor, and also lacked blood factorRhA(type). The child's blood was of the same type as its mother, namely,Rh0variant and lacking blood factorRhA(type). The possible genetic explanations for these observations and their clinical significance were discussed.