The complex serologic behavior of the Rh-Hr agglutinogens has been underlined by the recent discovery that, with very rare exceptions, associated with blood factor Rho of Rh-positive blood, there are numerous other blood factors which may be designated RhA, RhB, RhC, etc. Rare Rh-positive individuals exist whose bloods have blood factor Rho but 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 factor RhA, anti-RhA may be produced. When RhB and/or RhC are lacking, anti-RhB and anti-RhC may be produced. In fact, we have identified all three antibodies. These three antiserums are indistinguishable from "standard" anti-Rho serum in parallel tests on a random series of blood specimens unless one of the rare Rh-positive bloods lacking blood factor RhA, RhB or RhC are included.

In the present paper, anti-RhA serum from a sensitized type $$Word$$ mother whose child had erythroblastosis was used for studies on the distribution and heredity of the RhA blood factor. A total of 2012 blood specimens from Rho-positive individuals were tested; 951 from Caucasoids and 918 from Negroids. In tests on blood from Caucasoids, a "standard" Rho blood factor was invariably associated with a "standard" RhA blood factcr. In no instance where the reactions with anti-Rho serums were typical was the RhA factor absent or a variant. However, if the Rho factor was a variant, three possibilities with regard to factor RhA were identified. Either factor RhA was "standard," or factor RhA was a variant, or factor RhA was absent. This last possibility rarely occurs in Caucasoids since in our series only one Caucasoid blood or 0.1 per cent lacked RhA blood factor and in that case the Rho factor was a variant.

Among the 918 Rh-positive blood specimens from Negroids examined the situation was found to be somewhat different. While a "standard" Rho blood factor was almost always associated with a "standard" RhA blood factor, in 0.9 per cent the RhA factor was absent. Among blood specimens with a Rho variant blood factor, just as with Caucasoids, all three possibilities were identified, namely, bloods with "standard" RhA blood factor, with RhA variant blood factor and also bloods with blood factor RhA absent. The incidence of Rh-positive bloods lacking factor RhA was considerably higher among Negroids than among Caucasoids, namely, 1.6 per cent in Negroids, as compared with only 0.1 per cent in Caucasoids.

One interesting family was studied. The father’s blood had the "standard" Rho blood factor, but lacked the RhA component (type $$Word$$). The mother’s blood had the Rho variant blood factor, and also lacked blood factor RhA (type $$Word$$). The child’s blood was of the same type as its mother, namely, Rho variant and lacking blood factor RhA (type $$Word$$). Possible genetic explanations for these observations and their clinical significance are discussed.

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