Lymphocyte proliferation, as measured by incorporation of tritiated thymidine (3H-TdR), was significantly enhanced (p less than 0.01) when macrophages sensitized by target myeloblasts were added to monocyte-depleted lymphocyte fractions in the mixed leukocyte reaction (MLR) with human leukemic myeloblasts as stimulators and panels of normal lymphocytes as responders. Monocyte addition in the same concentration range to unfractionated lymphocytes resulted in highly significant facilitation (p less than 0.0001) of MLR response patterns to myeloblastic stimulation. However, with substitution of a different myeloblastic stimulator, this facilitation was not observed. At higher monocyte-lymphocyte ratios (1:15) the monocytes appeared to be capable of strongly inhibiting the MLR. Monocyte capacity to engulf and kill Candida albicans organisms was normal in acute myeloid leukemia (AML) patients given "immunotherapy" with BCG and leukemic cells.