alpha-L-Fucosidase (EC 3.2.1.51; FUS) activity and isoenzyme characteristics were analyzed in normal lymphocytes, normal (polymorphonuclear leukocytes, PMNs), and myeloid and lymphoid leukemic cells. Chronic lymphocytic leukemia (CLL) lymphocytes had a lower mean specific activity than normal lymphocytes (2.5 vs. 4.0, p less than 0.05). Acute lymphoblastic leukemia (ALL) blasts had a higher mean specific activity compared to normal lymphocytes (9.7 vs. 4.0; p less than 0.001), CLL lymphocytes (9.7 vs. 2.5; p less than 0.001), and acute myeloid leukemic (AML) blasts (9.7 vs. 7.6; p = NS). Normal PMNs had a higher mean specific activity than normal lymphocytes (7.0 vs. 4.0; p less than 0.05) but similar activity when compared to CML cells or AML blasts. Blasts from acute myelomonocytic leukemia (AMMoL) patients had higher activity than normal PMNs (9.0 vs. 7.0; p greater than 0.05). The isoenzyme patterns of normal and leukemic granulocytes and lymphocytes were obtained by automated chromatofocusing on PBE-94 microcolumns with normal and leukemic lymphocyte lysates. With normal and leukemic lymphoid lysates two major isoenzyme components (B and A) were isolated. The isoenzyme patterns of PMN, AML, CML, and AMMoL revealed three major peaks (B, A, I), totally different from those seen in lymphoid cells. The patterns of AML, CML, and PMN appeared to be similar to each other; however, the isoenzyme pattern obtained from AMMoL cells could be distinguished from the others by a prominent I peak. Thus, the FUS isoenzyme profile distinguishes the blasts of AMMoL from AML; and AMMol and AML from ALL.