Ten red cell samples lacking the high incidence Gya antigen were found to have the previously undescribed Do(a-b-) phenotype. Fifteen Hy- red cell samples were Do(a-b+) with weak expression of Dob and 6 Jo(a-) red cell samples were Do(a+) with weak expression of Doa. Five of the 6 Jo(a-) samples had extremely weak expression of Dob. The sixth Jo(a-) was Do(b-). Immune precipitates were prepared from radio-iodinated antigen-positive red cells with anti-Gya, -Hy, -Doa and Dob. Immunoblotting of these immune precipitates with affinity-purified anti-Gya and anti-Dob under non-reducing conditions revealed similar broadly migrating bands of M(r) 48,700-59,750, suggesting that the Doa and Dob antigens are carried on the same glycoprotein as Gya and Hy. The phenotypically associated high incidence Joa antigen has previously been shown to reside on the Gya/Hy glycoprotein. Enzyme-treated and chemically modified red cells tested with anti-Doa, -Dob, -Gya, -Hy and -Joa gave the same pattern of reactivity. We propose that Gya, Hy and Joa become part of the Dombrock blood group system and that, henceforth, the Gya/Hy-active glycoprotein be renamed the Dombrock-active glycoprotein. The Gy(a)-Hy- Jo(a-) phenotype constitutes the 'null' phenotype within the Dombrock system.