Erythrocytes from neonates with elliptocytosis were studied for their pattern of heat-induced fragmentation. Membrane alterations began at 44 degrees C. There was a gradual progression in shape changes as the temperature was increased to 47 degrees C, at which point frank fragmentation occurred. Normal erythrocytes show no morphologic changes until the critical temperature of fragmentation, 49 degrees C is reached. Heat studies were repeated a few months later, at a time when the patients' erythrocyte morphology had become typical of elliptocytosis. Morphologic changes occurred abruptly at 48 degrees C with complete fragmentation. Increased thermal sensitivity of the red cell membrane has previously been demonstrated for pyropoikilocytes, and these studies suggest that some cases of elliptocytosis may be mild expressions of a similar membrane defect.