Activation of mouse peritoneal macrophages during infection of mice by various facultative intracellular bacteria and after intravenous injection of recombinant interferon-gamma (rIFN-gamma) was studied. Macrophage activation was demonstrated on the basis of three different criteria, i.e. inhibition of Toxoplasma gondii proliferation, enhanced release of H2O2 and increased expression of Ia antigen. Macrophages activated during an infection with Salmonella typhimurium showed no enhanced salmonellacidal or listericidal activity relative to control macrophages, whereas Listeria-activated macrophages killed Listeria but not Salmonella faster than control macrophages. The rate of proliferation of Salmonella in spleen and liver of activated mice was comparable to the proliferation in the organs of control mice. rIFN-gamma-activated macrophages displayed neither an enhanced salmonellacidal nor an enhanced listericidal activity. When high numbers of Listeria were injected intravenously the proliferation in spleen and liver of rIFN-gamma-treated and control mice was similar. The proliferation of Listeria in the liver of rIFN-gamma-treated mice was less than in control mice when 1 LD50 or lower numbers of bacteria were injected. It is concluded that peritoneal macrophages become activated during infections of mice with various intracellular pathogens. However, these activated macrophages do not show enhanced bactericidal activity against all bacteria. Furthermore, rIFN-gamma is not sufficient to enhance the listericidal activity of macrophages.