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Abstract. The CFU‐s proliferative potential varied greatly during long‐term cultivation. Most of the CFU‐s in the cultures were represented by cells with low renewal capacity. Pre‐CFU‐s cells capable of producing multipotential colonies in methylcellulose, which contained CFU‐s with a high proliferative potential, were identified in the culture. In cultivation of a mixture of cells of different karyotype their ratio changed rapidly from week to week. the findings were consistent with the hypothesis that haemopoietic stem cells are maintained in the culture by the products of a small number of clones which arise and decline in succession, and that pre‐CFU‐s, but not the CFU‐s themselves, are clonogenic progenitors.

Stimulation of endogenous and exogenous colony formation by antilymphocyte serum has been observed. The effect of ALS on endocolonization is associated with promotion of CFU recirculation. ALS‐induced stimulation of exocolonization was observed only with adult bone marrow cells of normal mice. ALS caused no effect on colony formation induced by embryonic liver cells or rapidly proliferating haemopoietic cells of early radiation chimeras. ALS did not cause an increase either in the content of CFU in the spleen or in their proliferating fraction. It is assumed that the ALS effect is exerted on the microenvironment and not directly on the CFU, as the result of which short‐range regulation of haemopoietic stem cell changes.

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