The contribution of leukocyte apoptosis to the resolution of meningeal inflammation in bacterial meningitis was studied in the cerebrospinal fluid (CSF) and in meningeal infiltrates of humans and rabbits by in situ tailing, flow cytometry, agarose gel electrophoresis, and electron microscopy. In humans, the rate of apoptotic granulocytes was 21.0+/-20.8% (n=11) in cytocentrifuge preparations and 3.3+/-3.4 (n=14) in putrid infiltrates of autopsy cases (P=0.02). In rabbits, CSF pleocytosis peaked 8 h after the initiation of antibiotic treatment (5311+/-3122/microl). At this time, the rate of apoptotic granulocytes was 15.2+/-7.3% in CSF and 1.8+/-1.4% in the meningeal infiltrates (each group n=6, P=0.007). Thereafter, the rate of apoptotic granulocytes in CSF declined below 10%. In humans and rabbits, bands representing internucleosomal fragments of approximately 180 base pairs and multiples thereof were documented on agarose gels. Phagocytosis of apoptotic granulocytes by macrophages was visualized by light and electron microscopy. In conclusion, during resolution of subarachnoid space inflammation in bacterial meningitis, a substantial fraction of granulocytes undergoes apoptosis. These granulocytes are removed by phagocytosis by macrophages. Apoptosis is more frequent in granulocytes floating in the CSF than in adherent cells.