CD56 antigen (detected by NKH-1) is distributed on NK cells, monocytes, and ectodermal neural cells. In this study, the blasts of 29.2% of 27 patients with acute nonlymphocytic leukemia (ANLL) expressed CD56 antigen, but not CD16, CD2, or CD3 antigen. Leukemic cells isolated from 3 patients with CD56-positive ANLL did not have NK activity. There were no significant differences between CD56-positive and CD56-negative ANLL in CD13-positive cases, CD33-positive cases, and HLA-DR-positive cases. These results suggest that CD56-positive ANLL could be so-called mixed-lineage leukemia (lymphoid-associated antigen in ANLL).