The leukemia from which the human cell line HL-60 was derived was classified in 1976 as acute progranulocytic leukemia (APL), although it was recognized to show a number of atypical features. In the ensuing 10 years, the concept of APL and its integral association with t(15;17) has evolved, and the concept of APL as a morphologically recognizable entity has become embodied in the term French-American-British classification M3 (FAB-M3). It is now recognized that not every case of leukemia with a high proportion of progranulocytes can be classified as FAB-M3. We reviewed the light and ultrastructural morphology of the original diagnostic material from this case, and we report that the leukemia from which HL-60 was derived does not conform to the currently recognized entity of FAB-M3 and is more appropriately classified as an acute myeloblastic leukemia with maturation, FAB-M2.