Investigations were made on the influence of lipids from Listeria monocytogenes upon the course of infections induced by bacteria and fungi. Chloroform-methanol extract of the lipids was given to animals in various doses and at various intervals before and after infection. Mortality of mice and elimination rate of bacteria from the blood and organs were estimated. It was observed that the lipids markedly increased natural immunity of the animals against bacterial and fungal infections. The mortality rate of mice was decreased and the elimination of bacteria and fungi from the tissues was accelerated. Our suggestion is that the observed effect of lipids probably depends on the stimulation of mononuclear phagocyte system.