An antibody found in the serum of a Japanese blood donor detects a new high-frequency red cell antigen named UMC. UMC is absent from Inab phenotype cells, weakly expressed on Dr(a-) cells, and destroyed by alpha-chymotrypsin but not trypsin treatment of red cells. UMC is, therefore, a Cromer-related antigen. Immunoblotting with anti-UMC showed that UMC antigen, in common with other Cromer-related antigens, is carried on a red cell surface glycoprotein with an apparent molecular weight of 70,000, which presumably is the complement regulatory protein decay-accelerating factor.