Mucopolysaccharide sulfation was demonstrated in leukemic granulocytic precursors in short-term suspension culture by incorporation of 35SO4 into a compound identified chromatographically as chondroitin sulfate. The gel filtration pattern of sulfated mucopolysaccharide obtained from leukemic leukocytes was qualitatively similar to that found with normal granulocytic precursors. Sulfation of mucopolysaccharide was about 50% of the normal level in cells from chronic granulocytic leukemia, and approximately 15% of the normal level in cells from acute granulocytic leukemia or chronic granulocytic leukemia in blastic crisis. Lymphocytes from acute and chronic lymphocytic leukemia showed only traces of sulfate incorporation. Leukocyte mucopolysaccharide sulfation was studied in cultured cells from patients with acute leukemia to determine whether this reaction could aid in distinguishing acute granulocytic leukemia from acute lymphocytic leukemia. Significant levels of sulfation were obtained in cells from all leukemias judged to be acute granulocytic, while virtually no incorporation was found in cells from acute lymphocytic leukemia when a 24-hr incubation period was employed. In studies employing a 3-hr incubation, agreement as to the cell of origin of the leukemia and the degree of sulfation in the intracellular fraction was obtained in 16 of 18 determinations performed on 16 patients with acute leukemia. It is proposed that determination of 35SO4 incorporation in the intracellular fraction may help to differentiate acute granulocytic and acute lymphocytic leukemia.