Cefotaxime concentrations obtained in the C.S.F. of twelve children suffering from bacterial meningitis and undergoing monotherapy with this antibiotic are reported. Among these 12 patients, 4 infants (aged 3 to 28 days) had neonatal meningitis (due to Serratia marcescens, Proteus mirabilis, Enterobacter cloacae, Escherichia coli); one infant (2 months old) had meningitis due to Salmonella panama; 5 children (aged 5 to 11 months) had meningitis due to Haemophilus; and 2 children had belated superinfection caused by a ventriculo-peritoneal shunt due to Klebsiella pneumoniae and Pseudomonas aeruginosa. Cefotaxime concentration reached a high level as early as one hour after the injection (3 to 19 mcg/ml), remained at this level until the fifth hour (1,8 to 14,3 mcg/ml) and decreased without significant proportionality with the disappearance of the inflammatory symptoms. Compared to the M.I.C. of the bacteria which caused the twelve cases of meningitis, these results show that the concentrations in the C.S.F. are much higher than the M.I.C.'s. These results are comparable to those of previous studies. Cefotaxime diffuses in the C.S.F. and gives concentrations which ensures an antibacterial activity that ampicillin could not reach: in particular against Haemophilus influenzae and enterobacteriaceae.