~~ ~Bacterial growth and lethality of Listeria monocytogenes in mice were augmented by carrageenan-treatment and X-irradiation (8 J kg-l), whereas growth and lethality of Pseudomonas aeruginosa were augmented by X-irradiation but not by carrageenan-treatment. Protection against L. monocytogenes, at least in the early phases, appears to depend mainly on macrophages, since carrageenan depletes macrophages but not polymorphonuclear cells (PMN), whereas protection against P . aeruginosa appears to depend mainly on PMN.Ineffectiveness of PMN in elimination of L. monocytogenes is supported by histological examination and observation of intracellular killing in vitro.