Bone marrow cells from a number of animal species have been used extensively in liquid and semisolid cultures to study hemopoiesis and to produce functional mature cells and factor-dependent cell lines (for review see ref. 1 ). Neutrophils and macrophages are produced without added growth factors from murine long-term bone marrow cultures (2 ), while lymphoid cells (3 ,4 ) and megakaryocytes (reviewed in ref. 5 ) can be induced under certain conditions.