We have investigated a 'new' low frequency antigen JAHK, which is a marker for the rare Rh gene complex rG. The rG haplotype does not produce any D, c or E antigens, but does produce a strong G antigen. The rG haplotype [d(C)(e)G] is associated with weak C and weak e antigens. Three unrelated rG/dce individuals and one rG/rG propositus were JAHK+. In addition, three propositi whose red cells had a typical expression of C and/or e antigen, which could not be shown to be rG because of a normal D antigen produced by the haplotype in trans, were also JAHK+. Families of three of the propositi demonstrate the inheritance of JAHK as a Mendelian dominant character. It is likely that the JAHK antigen results from conformational changes in an RhCcEe protein that has the amino acid characteristic of c antigen at position 16 and the amino acid residues characteristic of C antigen at positions 60, 68, and 103. JAHK has been assigned the number RH53.