The experiments described in this paper were performed in order to elucidate a possible regulatory role of acute and chronic myeloid leukemic leucocytes. The influence of these cells on either activated or non-activated mouse bone marrow cells was measured, and the results were compared with the action of the two major leucocyte fractions, mononuclear cells and granulocytes. It was demonstrated that acute leukemic blasts were almost completely unable to activate myeloid committed stem cells (Colony Forming Units, CFUc). In contrast, blasts from patients with chronic myeloid leukemia (CML) in blastic crisis disclosed a variable pattern of activation, in that two out of three cases exerted normal stimulatory capacity. A possible inhibitory action of leukemic cells was the subject of further experiments. Acute leukemic blasts in some cases disclosed inhibitory activity, a few cases appeared to be without influence on normal proliferating CFUc. Granulocytes from patients with CML in the chronic phase showed inhibitory capacity when compared with granulocytes from normal individuals.