Culture supernatant fluids from the human T-cell leukemia virus-positive cell line C10/MJ-2 were found to contain a soluble factor with macrophage-activating factor (MAF) activity. The MAF activity of this culture supernatant fluid was stable at 100 degrees for 2 min and was unaffected by treatment with human anti-gamma-interferon (IFN-gamma) monoclonal antibody. Both treatments neutralized greater than 97% IFN-gamma activity from the supernatant fluid as measured by virus neutralization. Furthermore, this MAF activity was not due to contamination with endotoxins since the Limulus amebocyte lysate assay was negative (less than 0.125 ng/ml) and preincubation of the C10/MJ-2 supernatant fluid with polymyxin B did not diminish its activating potential. By contrast, human IFN-gamma rendered human monocytes tumoricidal only when combined with Salmonella typhosa lipopolysaccharide (LPS) at a minimum dose of 0.2 ng/ml. The concentrations of both LPS and IFN-gamma were crucial to achieve activation since IFN-gamma at doses less than 10 units/ml did not activate human monocytes even when combined with maximal doses of LPS (0.5 ng/ml). Finally, when human IFN-gamma admixed with LPS was preincubated with polymyxin B, its activating potential was completely abrogated. Collectively, these data suggest that the human T-cell line C10/MJ-2 constitutively produces a diffusable product distinct from IFN-gamma which activates human monocytes to lyse tumor cells. Thus, this cell line could provide a good source of a unique human MAF for future purification procedures.