Erythrocyte membrane mucoid composition has been quantitated in 13 normal subjects, 11 subjects with Coombs’ negative hemolytic or blood-loss anemia with reticulocytosis and 7 subjects with acquired autoimmune hemolytic anemia. The stromal protein value was increased in all subjects with anemia and reticulocytosis. The increase in protein paralleled the reticulocyte count. An abnormality of the erythrocyte mucoid composition was observed in acquired autoimmune hemolytic anemia as evidenced by a significant decrease in the fucose to protein ratio. A lesser change was observed in the sialic acid to protein ratio. It is speculated that these changes in membrane mucoid composition may be related to the selective modification of certain red cell, fucose-containing receptors by endogenous enzymes, possibly activated by antigenantibody interaction.