Patients with acute myelogenous leukemia in remission have pronounced deficiency in antibody-dependent cellular cytotoxicity (ADCC) and mitogen-induced cellular cytotoxicity. The deficiency in ADCC was partly explained by reduction in the number of circulating effector cells (Fc receptor-bearing cells) demonstrable at a time when white blood cell and platelet counts were normal. These cytotoxic functions, as well as the circulating numbers of T-cells and Fc receptor-bearing cells were further decreased by the administration of monthly cycles of combination chemotherapy with 1-beta-D-arabinofuranosylcytosine and 6-thioguanine. Following each cycle of chemotherapy, progressive recovery of these functions occurs during the third and fourth weeks with occasional increases above base line in patients in whom chemotherapy is withheld for longer than five weeks. In selected patients recovery of one cytotoxic function preceded the other, indicating that these functions are mediated by different effector cells. Administration of a single dose of daunomycin i.v. had no effect in either of these cytotoxic functions or in the circulating numbers of lymphocytes. The decrease in ADCC effector cell function induced by phase cycle-specific agents correlated with the level of reactivity exhibited by patients after achieving bone marrow and clinical remission. Patients showing low levels of reactivity postremission experienced highest degree of depression. In two patients, complete abrogation of ADCC effector function was demonstrated with minimal recovery even six weeks after stopping chemotherapy. These findings indicate that effector cells in ADCC and mitogen-induced cellular cytotoxicity are highly susceptible to phase cycle-specific agents, and their recovery takes longer that of other lymphoid and nonlymphoid populations.