Ultrastructural changes of the bone marrow in chicks inoculated with chicken anaemia agent (CAA: MSB1-TK5803 strain) and age-matched controls are described. Erythropoiesis in the lumen of the intravascular spaces (sinuses) of unaffected bone marrow were classed into four consecutive stages according to cell morphology: proerythroblasts, basophilic erythroblasts, polychromatophilic erythroblasts and erythrocytes. In the early stage of infection, haematopoietic cells showed irregular plasma membrane, vacuolisation, formation of pseudopods and electron-opaque regions in the cytoplasm as well as intranuclear inclusions consisting of fine granular or homogeneous materials. Aggregation of virus-like particles, of 14 nm in diameter, was rarely observed in the degenerative haematopoietic cells. Subsequently, an increase in numbers of macrophages with engulfed degenerative and necrotic haematopoietic cells and plasma cells with prominent Golgi complex and well developed rough endoplasmic reticulum was observed in the anaemic stage. Although immature cells were rarely seen on day 12 of postinoculation (pi), active erythropoiesis resumed on day 20 pi or later.