The agglutinability of red blood cells from patients with various hematologic disorders was compared with that of erythrocytes from normal persons by quantitative hemagglutination in the ABH blood group system. Red blood cells from six patients with hypoplastic anemia or metastatic marrow disease as well as normal reticulocytes (five experiments) were less agglutinable than normal mature cells. Erythrocytes from one patient with lymphoma and an acquired hemolytic anemia showed enhanced agglutinability. Our data suggest that changes in erythrocyte reactivity may be more common than suspected, although usually undetected by less sensitive methods.