The erythropoietic cells of six cases of beta-thalassaemia trait were studied by electron microscopy and electron microscope autoradiography. Intracytoplasmic and intranuclear alpha-chain precipitates were found in some late polychromatic erythroblasts and intracytoplasmic precipitates were found in several marrow reticulocytes. This provides direct morphological evidence of unbalanced globin chain synthesis in the marrow. Several of the polychromatic erythroblasts and marrow reticulocytes contained autophagic vacuoles and showed a variety of other dyserythropoietic changes. Erythroblast profiles containing moderate quantities of precipitated alpha-chains usually suffered from a marked depression of protein synthesis. The proportions of marrow cells containing alpha-chain precipitates and displaying dyserythropoietic changes varied considerably from patient to patient. It is proposed that this variation largely reflects variations in the proteolytic capacity of the erythropoietic cells in different individuals and leads to different degrees of ineffective erythropoiesis in beta-thalassaemia trait.