The reduction of resistance against Salmonella infection was resulted from the reduction of macrophage activities in Sarcoma 180-bearing mice. Mean survival day of S180-bearing mice was markedly reduced to 1.4 +/- 0.5 days after the challenge with 1 X 10(8) of Salmonella typhimurium LT2 cells in contrast with 6.8 +/- 1.9 days of normal mice. Eighty percent of S180-bearing mice died within 14 days after the challenge with 1 X 10(6) of LT2 cells, whereas all of normal mice survived more than 30 days after the challenge with 3 X 10(6) or less of LT2 cells. Chemotactic and O2- producing activities of macrophages collected from S180-bearing mice which were treated with Salmonella mini-cells restored to normal level. Bactericidal activity of macrophages was also restored by the mini-cell treatment. The restoration of the resistance to Salmonella was accompanied with increases of macrophage activities.