The penetration of mecillinam, a new &bgr;-lactam penicillin-like antibiotic, into cerebrospinal fluid (CSF) was investigated in 11 children in whom all but 1 was presumed to have non-inflamed meninges. 30 minutes after a single intravenous dose of 30mg/kg, a concentration of approximately 0.30μg/ml was achieved in the CSF (corresponding mean serum concentration 75μg/ml), and this level was maintained during the next 4 hours in spite of rapidly declining serum concentrations, indicating a slow equilibration of mecillinam over the blood-liquor barrier. I child with suspected inflamed meninges showed a much higher CSF level of 12.1 μg/ml. It is concluded that mecillinam, as is the case for other penicillins, apparently crosses non-inflamed meninges poorly.