Peritoneal macrophages elicited by Lactobacillus casei YIT9018 (LCEPM) were incubated in culture for 18 h with L. casei; the culture supernatant (LCM) was then harvested and tested for its ability to increase the cytostatic activity of resident peritoneal macrophages (RPM) and LCEPM. Treatment of RPM with LCM induced activation of macrophages to a cytostatic state against L929, Colon 26, P815, P388D1 and L1210 cells. A combination of recombinant human tumor necrosis factor (rhTNF), recombinant mouse TNF (rmTNF), recombinant human interleukin-1 (rhIL-1) or bacterial lipopolysaccharide with recombinant mouse interferon gamma (rmIFN-gamma) resulted in the synergistic induction of cytostatic activity in RPM. Recombinant mouse granulocyte-macrophage colony-stimulating factor (rmGM-CSF) plus rhTNF increased the cytostatic activity of RPM a little but rmGM-CSF or rhTNF combined with rhIL-1 or alone had no effect. The effect of LCM on RPM was not inhibited by polymyxin B, anti-mTNF antiserum or below 20 U/ml monoclonal anti-rmIFN-gamma antibody (anti-rmIFN-gamma) but was inhibited by more than 40 U/ml anti-rmIFN-gamma, and LCM did not have any interferon antiviral activity. These results suggest that the cytostatic activity of RPM was augmented by the LCM, and that the effect of the LCM may be not due to IFN-gamma, TNF, GM-CSF, IL-1 or a small amount of contaminating lipopolysaccharide.