The behaviour of normal haemopoietic stem cells of the bone marrow during the development of the acute transferable rat leukaemia, L 5222, has been investigated. The granulopoietic committed stem cells, measured by the in vitro colony technique, showed a marked decrease to less than half normal levels. Pluripotent stem cells included in the small lymphocytes of the bone marrow, and labelled with 3Hthymidine by the complete labelling method, showed only a modest decrease in number and an unchanged labelling intensity. The results suggest that in this leukaemia the pluripotent stem cells may be affected in such a way that they are unable to react by proliferation to the depletion of the succeeding cell compartments. This might be due to inhibition by leukaemic cells or to a disturbed feedback regulation between the committed and pluripotent stem cell compartments.

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