Two examples of an autoantibody that defines a hitherto unrecognized Rh system antigen are described. Both were produced by C-negative individuals and ostensibly resembled anti-C in specificity. However, adsorption studies showed that the antigen that the autoantibodies define is present on all red blood cells with a “normal” Rh phenotype and on D—/D— and Dc-/Dc- samples. The antigen detected is not present on Rhnull red blood cells. Serologic studies have shown that the new antibody, that has been named anti-Rh39, has a different specificity from those that define the antigens C, Ce(rh1), G, Hro, Hr, CG, LW, Rh:29, Rh:34 and U. A possible relationship between auto-anti-Rh39 and alio-anti-C, in terms of the immune response, is discussed.