Simian antisera to human leukemia cells were able to distinguish antigens specific for lymphocytic types of leukemia from those expressed on certain myeloid leukemia cells. In this investigation, cells from acute myelomonocytic leukemia patients (AMML) were examined for their membrane-associated leukemia antigens. Simian antisera to both lymphocytic and myelogenous leukemia cells lysed cells from AMML donors. Monkey antisera to AMML cells, by direct microcytotoxicity testing, were cytotoxic for cells from all AMML patients, as well as for cells of certain patients with myeloid leukemia. Cells from patients with lymphatic leukemia were nonreactive. However, absorption studies indicated an antigen present on cells from patients with chronic lymphocytic leukemia which cross-reacted with AMML cell antigens. Sequential analyses of the serologic reactivity of cells from AMML patients undergoing chemotherapy corresponded with the clinical course of the patient, even though there was little correlation between the percentage of blast cells present and the per cent cytotoxicity with the antisera. At certain times a higher percentage of seropositive cells could be detected over that seen on morphological evaluation. The estimation of leukemic cells by serologic means could aid in the diagnosis and management of AMML patients during chemotherapy.