Electron microscope studies have been performed to investigate the ultrastructural basis of the characteristic golf-ball-like light microscope appearance which develops when the erythrocytes of patients with HbH disease are stained supravitally with brilliant cresyl blue or new methylene blue. The data indicate that the golf-ball appearance resulted from the formation of many spherical or biconvex masses of electron-dense material which were attached to and which bulged the cell membrane. These masses presumably consisted of denatured HbH. A variable proportion of the red cells of the four patients investigated failed to form such membrane-associated inclusions even after treatment with redox dyes for 24 h. Studies of subpopulations of red cells which were separated according to cell age suggested that there was a considerable variation in the HbH content of young red cells and indicated that with increasing cell age there is an increase in the proportion of red cells which do not contain appreciable quantities of HbH.