Some aspects of erythroblast function have been investigated in two Thai patients with haemoglobin H disease. Electron microscope and high resolution autoradiographic studies revealed that (1) some erythroblasts and marrow reticulocytes contained branching intracytoplasmic inclusions and (2) the presence of such inclusions did not necessarily prevent the entry of cells either from the G1 phase into the S phase or from the G2 phase into mitosis. The distribution of early polychromatic erythroblasts in the different stages of the cell cycle was determined by the technique of combined Feulgen microspectrophotometry and 3H-thymidine autoradiography and found to be essentially normal. It is concluded that although the presence of an excess of alpha-chains in the beta-thalassaemia syndromes has been previously shown to be associated with a gross impairment of erythroblast proliferation, the presence of an excess beta-chains in HbH disease is not.