After the cytology and the chemistry of the CSF had returned to normal, the concentration of ceftizoxime was determined in the CSF and in the serum of 27 patients who had suffered from purulent or serous meningitis. The patients were divided into four groups. The concentration in the CSF was mean = 0.87 mg/l two hours after a bolus injection of 2 g ceftizoxime. After an infusion of 2 g ceftizoxime over 30 minutes, the concentration in the CSF was mean = 0.32 mg/l one hour and mean = 0.99 mg/l two hours after the infusion was started. The highest concentrations in the CSF were obtained six hours after a bolus injection of 2 g ceftizoxime. The peak value was 4.3 mg/l and the mean 1.78 mg/l. Based on these pharmacokinetic data, additional injections could have a cumulative effect on the concentration of ceftizoxime in the CSF. With the exception of Staphylococcus sp., Pseudomonas sp. and some anaerobes, it can be expected that ceftizoxime will be effective against other organisms causing meningitis. The results of clinical applications remain to be seen.