We have previously reported long term erythroid differentiation of adult bone marrow cells seeded onto adherent cells derived from adult bone marrow. In this paper, we show that the adherent cells obtained from foetal liver do not support the erythroid differentiation of either adult bone marrow cells or foetal liver cells. Adherent layers derived from bone marrow of adult W/Wv mice supported differentiation of adult bone marrow precursors, but foetal liver progenitors only produced erythrocytes for a few weeks and the foetal origin of these red cells was confirmed by haemoglobin typing. The duration and extent of erythropoiesis was generally inversely proportional to the cell dose. Foetal progenitors were as sensitive to erythropoietin as adult cells, but were optimally stimulated at a lower plateau concentration. These results suggest that inhibitory cells present in foetal liver may block erythropoiesis and their growing importance with age may provide an explanation for the arrest of erythropoiesis in the liver at late developmental stages.