Haemopoietically active long-term bone marrow cultures from several species have been investigated ultrastructurally. Human, tree shrew and mouse cultures generally support granulopoiesis, although recently it has been possible to convert a granulopoietic mouse culture to extensive erythropoiesis. The haemopoietic products of the cultures include granulocytes (neutrophil and basophil), mast cells, monocytes, megakaryocytes and all stages of the erythrocytic series. Plasmacytes and occasional lymphocytes have been observed in small numbers in human cultures (possible indicating retention rather than formation). The stromal elements of the adherent layer of these cultures include endothelial cells, reticulum cells, fat cells and fibroblasts. The adherent layers are responsible for the inductive microenvironment within the cultures, and show features specific for the line of differentiation. In the granulocytic cultures there is close association between developing fat cells (reticulum cells) and granulocyte precursors. Endothelial cell monolayers cover large regions of these cultures, and the areas beneath this monolayer are rich in early granulocytes. Mature granulocytes and monocytes migrate through the endothelial layer, demonstrating in vitro "transmural passage". Cultures stimulated for erythropoiesis show a considerable reduction in fat cells, in endothelial cell cover and in the numbers of classical monocytes. Erythropoiesis appears to be promoted by a close association of the entire erythrocytic series with monocytic cells, forming "erythroblastic islets" in vitro. A possible pathway of intracellular communication between differentiating haemopoietic cells and the stromal cells in their microenvironment is suggested.