Dispersed and intact marrow fragments have been implanted in the omentum and subcutaneous fat of irradiated and nonirradiated hosts. Osteogenesis and hemopoiesis in varying proportions were observed at 10 and 35 days. The subcutaneous site favored osteogenesis, the omental site and host irradiation prior to transplant resulted in predominance of hemopoiesis. In dispersed and recompacted marrow grafts, only hemopoiesis developed. Differentiation was predominantly granulocytic. Osteogenesis is not an obligatory prerequisite of hemopoiesis in extramedullary sites.