The expression of two myeloid antigens identified by the monoclonal antibodies (McAb) MCS2 (CD13) and MY9 (CD33) was investigated in 136 cases of leukaemia. MCS2 was positive in blast cells of 78 of 88 (88.5%) and MY9 in 51 of 81 (64%) cases of acute myeloid leukaemia (AML) and chronic granulocytic leukaemia in myeloid blast crisis. One or other McAb, or both, were positive in all but 2 (2.3%) of these cases. MCS2 was more sensitive than MY9 to detect blasts of the myeloid lineage due to its most frequent reactivity in the cytoplasm of fixed cells by the immunoperoxidase (IP) technique compared with its membrane expression on cell suspensions by immunofluorescence (IF). MY9 was not suitable for tests on fixed cells. MCS2 was positive by IP but not by IF in 24% of AML, but the reverse was not observed. This suggests that the antigen detected by MCS2 is expressed in myeloblasts first in the cytoplasm and later on the cell membrane, pattern which is similar to that of the early antigens CD3 and CD22 in T and B lineage lymphoblasts, respectively. MCS2 was always positive in FAB types of AML-involving myeloblasts (M1-M4), including cases of undifferentiated morphology (M0), whilst MY9 was more frequently positive in monocytic leukaemia (M5). On the other hand, MCS2 was positive in 4 of 33 cases of acute lymphoblastic leukaemia and MY9 in 1. We conclude that both McAb, particularly MCS2, contribute to the better characterisation of myeloid leukaemias but that other tests are required to clarify the nature of the blasts when unexpected reactivities are observed.