The contribution of phagocytes to protection against LiJteria monocytogenes was analysed in outbred ddN mice. Most of the bacteria injected intravenously at a dose of 3 x lo3 to 4 x lo3 were trapped in the liver within 10 min. There was a transient 10-fold decrease in the number of bacteria by 6 h. Anti-listeria activity in the initial phase was resistant to X-irradiation but was inhibited by carrageenan, and was not influenced by immunization. The protection in this very early stage of infection seemed to be attributable to the function of fixed macrophages. Viable bacteria in the organs increased progressively but slowly from 6 h to 72 h to reach maximum numbers. Bacterial growth during this period was markedly enhanced by X-irradiation or treatment with carrageenan. Accumulation of free phagocytes seemed to suppress the bacterial growth in this phase. The number of bacteria began to decrease from day 4 and became undetectable by day 9. The suppressive effect on bacterial growth in this last phase may be dependent on immunologically activated macrophages and was reversed by X-irradiation and carrageenan. The course of local infection was similar to that of systemic infection except for the lack of initial decrease. We conclude that the course of infection with L. monocytogenes can be divided into three phases with regard to the roles of phagocytes in resistance.

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