Long-term liquid cultures of mouse bone marrow produce stem cell (CFU-S) and differentiated granulocytes for many months. Addition of AMS (anaemic mouse serum) to the cultures almost entirely eliminates the granulopoietic activity and stimulates erythropoiesis, with full erythroid maturation and the production of adult haemoglobin. Ultrastructural analysis of in situ fixed material reveals the cell shape and surface morphology of the erythroid maturation series, and the generation of erythroblastic islands in vitro. Each erythroblastic island consists of one or more synchronously maturing cohorts of erythroid cells undergoing four or five divisions between proerythroblast and normoblast. Each island is centered on a macrophage, which interacts with the developing erythroid population in several ways. Expelled nuclei are phagocytosed by te macrophage, which also has large areas of closely apposed membrane with the erythroid cells, gap junctions, and possible reciprocal vesicular activity. Changes in the adherent layer (stromal cells) also occur with the transition from granulopoiesis to erythropoiesis. There is a reduction in the endothelial cell cover, and mobilisation of lipid from the granulopoietic associated apidocytes.