Out of 60 patients with acute myeloid leukemia not preceded by chronic myelocytic leukemia or any preleukemic phase, 7 had both lymphoid and myeloid markers. All patients expressed common acute lymphatic leukemia antigen (CALLA) in 20% or more of their leukemic cells, which also showed positive peroxidase reaction. In addition, 4 patients had Auer rods. Two additional patients had morphologically clear acute monocytic leukemia (FAB M5b) and cells expressing CALLA. In 4 of the 7 patients the sum of the percentages of peroxidase or Leu M1 + and CALLA-positive blast cells exceeded 100%, suggesting that at least some of the cells had both myeloid and lymphoid markers. Moreover, out of 3 patients where double staining with the CALLA antibody J5 and the myeloid marker Leu M1 was performed, 2 had both markers on the same cells.