Fucosyltransferase (FT) activity of normal lymphocytes, normal granulocytes, and various types of human leukemic cells and electrofocusing pattern of FT activity in human leukemic cells and normal lymphocytes were examined using asialofetuin as an acceptor. Levels of FT activity in normal lymphocytes were higher than those of normal granulocytes in which FT activity was almost undetectable. The FT activity was higher in blast cells of acute myeloblastic leukemia and chronic myelogenous leukemia in blast crisis than in blast cells of acute lymphoblastic leukemia and the chronic phase of chronic myelogenous leukemia. The level of FT activity was lower in cells of chronic lymphocytic leukemia than that of normal lymphocytes, but it was higher than that of normal granulocytes. Three main isoelectric forms of FT in leukemic blast cells were identified by isoelectrofocusing, and they each had a characteristic focusing point: around pH 4.5 (peak 1); pH 4.9 (peak 2); and pH 5.2 (peak 3). In blast cells of myeloid leukemia, the activity of peak 3 was markedly higher than those of peaks 1 and 2. In blast cells of lymphoid leukemia, the activity of peak 3 was also the highest, but the activity of peak 2 was higher than that in myeloid blast cells. In normal lymphocytes, the major isoelectric form of FT was focused at around pH 4.9 and peak 3 was undetectable. These results indicated apparent differences not only in FT activity but also in isoelectric forms of FT between myeloid leukemic cells and lymphoid leukemic cells.