Studies were undertaken to establish whether lymphokines affect bacteria in a direct and/or an indirect fashion via monocytes. Lymphokine-rich supernatants were generated by stimulating mononuclear cells with killed Bacillus subtilis bacteria. Various strains of bacteria were incubated in these culture supernatants and plated. Also, monocytes were incubated in the supernatants, after which their capacity to phagocytose and kill Staphylococcus albus was measured. It was found that the culture supernatants did not contain activities exerting a direct effect on bacteria. They did, however, contain factors activating monocytes. The uptake and the killing of staphylococci increased 91% and 68%, respectively, after three days' incubation in lymphokine-rich supernatants. The monocyte-activating principle eluted in Sephadex G-100 chromatography over a wide molecular weight range (greater than 68000 - less than 23000) was not inactivated at 56 degrees C and was reduced by L-fucose. It thus shared the characteristics of human MIF or MAF.