Tumor necrosis factor (TNF) is a cytokine with pleiotropic biological activity. We have undertaken the present study to evaluate a possible ability of peripheral blood cells of healthy subjects and patients with acute monocytic leukemia to produce TNF alpha. We studied 17 leukemia patients and 8 control subjects. Spontaneous production of TNF alpha induced by LPS and Newcastle disease virus (NDV) were determined in blood cell cultures. Leukemic cells released markedly higher level of TNF alpha than control cells cultured in vitro in contrast to control cells producing TNF spontaneously. It seems that leukemic cells can produce TNF in response to very small amount of LPS; however, we cannot exclude that spontaneous TNF may be involved in autocrine regulation of proliferation and differentiation of bone marrow monocyte precursors.