Ten patients with purulent meningitis received 3 intravenous injections of 4 g piperacillin at intervals of 8 hours, in addition to the usual antimicrobial treatment. Piperacillin was assayed in serum and CSF by high performance liquid chromatography. The mean CSF concentration of the drug was 9.2 micrograms/ml and its mean percentage of penetration was 22,7%. There were no significant differences in CSF concentrations between days 2 to 4 (inflamed meninges) and days 10 to 20 (patient cured). It is concluded that piperacillin shows good CSF penetration and could be useful to treat selected cases of meningitis due to Gram-negative bacilli.