Surface la-positive cells were found to vary from 0 to 100% in initial blood specimens from 37 adults with acute myelogenous leukemia (AML). When myeloblasts from 19 patients were tested against panels of lymphocytes from 5 to 19 normal donors, mean stimulation indices ranged from 1 to 60. Some leukemic myeloblasts strongly stimulated most allogenic responder lymphocytes whereas others produced almost no stimulation. The addition of antibody against human la to 28 mixed leukocyte reaction (MLR) combinations resulted in significant inhibition (p < 0.001) of 3 50% la-positive myeloblasts had a significantly longer survival than patients with fewer la-positive myeloblasts (p < 0.04).