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Irradiated mice infused with bone marrow cells developed colonies both in the spleen and on the macrophage layer of the cellulose acetate membrane which had been inserted into their peritoneal cavity. Factors influencing the ratio of spleen to macrophage-layer colonies were examined. The ratio of the colonies depended on the condition of the recipients rather than the condition of the injected cells provided there was no marked histoincompatibility between donors and recipients.

The colony-forming ability of haematopoietic cells of W anaemic mice was examined on the macrophage layer formed in the peritoneal cavity of mice. Bone marrow cells of W anaemic mice formed a considerable number of colonies on the macrophage layer, notwithstanding they did not form any colonies in the spleen of the same recipients. As the colony-forming ability of the bone marrow cells was not reduced by the incubation with 3-H-thymidine, most of the cells which formed colonies on the macrophage layer seemed to stay in G0 state. The interrelationship between the spleen colony-forming cells, the macrophage-layer colony-forming cells, and in vitro colony-forming cells was discussed.