A Japanese blood donor (H. T.) and his brother (M. S.) are the first homozygous MkMk individuals described; their red cells lack, as expected, known antigens of the MNSs blood group system and also have no demonstrable MN-active and Ss-active glycoproteins. Both MkMk individuals have a naturally occurring atypical antibody in their serum. The antibody in the serum of H. T. is inhibited by MNSs-active glycoprotein preparations from normal erythrocytes.