The natural killer (NK) cell activity of the blood and marrow was studied in patients with acute myelogenous leukemia (AML). NK activity of the cells from blood and marrow was significantly decreased in AML to three target cell lines. Both binding and killing capacities of the effector cells were deeply depressed in the blood as well as in the marrow at single cell assay. Surface phenotypic analysis showed a significant decrease in CD 11+ cell subsets, but not in CD 16+ or Leu-7+ cells, in both blood and marrow cells from AML. A significant decrease of large granular lymphocytes (LGL) was also displayed in these samples at morphological examination. The effector cells from AML patients poorly responded to interferon stimulation in NK cytolysis. Taken together, a decrease in CD 11+ cell population with LGL morphology appeared to be responsible for the impaired NK activity in patients with AML.