Intravenous infusion of carbenicillin was followed by a determination of the concentration of this antibiotic in both the liquor and serum. The examinations, of which the results are reported in this paper, were made on six patients who had undergone surgical operations to the posterior cranial fossa and on whom lumbar punctures were postoperatively performed every day, as well as on seven hydrocephalic patients where a temporary relief of pressure was obtained by draining excess liquor. The level of carbenicillin in the liquor were found to enable even gram-negative problem germs (e.g., Proteus germs) to be controlled.