We have used high performance liquid chromatography to evaluate the meningeal penetration of apalcillin. In subjects without meningitis the penetration was poor and the levels obtained did not exceed 1.75 mg/l. In cases of meningitis, when the spinal fluid albumin level was greater than 0.60 g/l, the levels varied from 5 to 30 mg/l. These figures are in accord with those obtained in cases of experimental meningitis. A dosage of 200 mg/kg/day should allow the treatment of meningitis due to sensitive bacteria and in particular due to Pseudomonas aeruginosa.