Summary. A liquid culture system for murine haemopoietic cells is described where proliferation of granulocytes (G-cultures) or mononuclear cells (M-cultures) can be maintained for many weeks. In M-cultures there is a rapid loss of spleen colony forming cells (CFUs). Agar colony forming cells (CFUc) are present for 6 weeks, then decline rapidly. Colonies formed from these CFUc contain only large mononuclear cells. In G-cultures there is proliferation of CFUs, grafts of which (after 7-9 weeks in culture) protect lethally irradiated mice from haemopoietic death. Similarly, there is proliferation of CFUc, which form colonies in agar consisting of granulocytes, in all stages of differentiation.