Total, cell-free, and cell-bound endotoxin and bacterial density were measured in cerebrospinal fluid (CSF) of 22 children with Hemophilus influenzae meningitis. Also the effect of ceftriaxone on CSF endotoxin levels was investigated in eight patients by reexamining their CSF 2-6 h after the initial dose. Initial CSF bacterial density correlated with initial CSF endotoxin levels (P less than .001). Ceftriaxone induced a marked increase of free endotoxin in CSF, from an initial (mean +/- SE) 0.75 +/- 0.21 to 1.29 +/- 0.23 log10 ng/ml (P less than .01). This increase correlated positively with the number of bacteria killed in the CSF (P less than .01). The increase in free endotoxin was associated with an increase in mean CSF lactate levels from 8.5 to 9.7 units/l (P less than .05) and mean lactate dehydrogenase levels from 102 to 180 mmol/l (P less than .02) and a decrease in mean CSF glucose from 1.17 to 0.46 mmol/l (P less than .05). Initial CSF total endotoxin concentrations correlated both with the Herson-Todd clinical severity score (P less than .001) and with the number of febrile hospital days (P less than .001). These findings suggest that highly bactericidal agents initially lead to release of free endotoxin from gram-negative organisms into CSF, with associated enhanced inflammatory response by the host.