Desacetylmethylcolchicine decreased the leukocyte count and produced reduction or disappearance of immature cells in the peripheral blood in all of 14 patients with chronic myelocytic leukemia. These effects were usually accompanied by regression of splenomegaly and hepatomegaly, rise in hemoglobin concentration, and subjective improvement. A beneficial effect was also observed in some cases of blastic crisis. This drug appeared to have greater selectivity of action against cells of the granulocytic series than other antileukemic agents. Continuous adminstration proved necessary to maintain remission. Regulation of dosage was difficult; hematologic relapse occurred with suboptimal doses and toxic manifestations occurred with doses above the optimum. The drug's principal advantages are: prompt therapeutic action, possible effectiveness against myeloblastic proliferation, and rapid reversibility of its toxic effects.