THE concept of the haemopoietic inductive microenvironment as first proposed by Curry and Trentin1 has been amply confirmed by experiment. One salient feature of this concept relates to the role of the mouse spleen in promoting erythropoiesis rather than granulopoiesis. For example, in the clonal growth of transplanted bone marrow cells, the spleens of the irradiated host mice have erythroid and granuloid colonies in a ratio of 3, whereas the ratio is 0.5 in the bone marrow of the same hosts2,3. Other data show that there is a positive effect of splenic tissue, rather than a negative effect of bone marrow, on erythroid colony growth3. We have tested this concept using cocultures of mouse spleen and bone marrow cells and have found that phenomena related to the haemopoietic inductive micro-environment can be studied in vitro.