The purpose of this report is to direct attention of hematologists to a mild chronic hemolytic anemia, of unexplained etiology, characterized by the remarkable absence of all Rh-Hr factors. There are also defects in the Ss, U blood group determinants. The anemia has most of the routine hematological characteristics of mild stomatocytosis and/or hereditary spherocytosis. Like hereditary spherocytosis, it shows no abnormalities of intraerythrocytic enzymes nor of membrane lipids. Unlike hereditary spherocytosis, however, its inheritance is recessive. The proposed basis for this hemolytic anemia is homozygosity for a gene whose normal allele produces components of the erythrocyte membrane which function in the maintenance of intracellular osmotic equilibrium and which serves as substrate for the action of both the products of the Rh-Hr genes and of the genes of the MNSs, U and, possibly, of the Fy systems.