The changes in mouse corneal inflammatory cell population and the phagocytic capability of these cells were studied quantitatively at 24 and 48 hours after eliciting inflammatory cells into the cornea byPseudomonas aeruginosainoculation. Mice were selected for study according to their ability (young adult Swiss-Webster) or inability (aged Swiss-Webster) to restore corneal clarity after bacterial inoculation. Inflammatory cells were recovered from enzymatically disaggregated corneas, nucleated cells counted, and cell viability assessed to be 95% by trypan blue dye exclusion. Polymorphonuclear neutrophilic leukocytes (PMN) and macrophages recovered in this manner showed no significant differences in cell population contribution to the differential leukocyte count, but did show significantly fewer phagocytically active cells in aged when compared with young adult mice. These phagocytosis data were confirmed in a separate study using peripheral blood cells obtained from uninoculated mice of each age. The impaired phagocytic function seen in aged mice may contribute to the failure to resolve corneal clarity observed in these animals afterPseudomonasinfection.