The cerebral penetration of ceftriaxone in patients who underwent surgery for cerebral tumours was investigated. Seventeen patients received 2 g of ceftriaxone given intravenously 2 to 13 h before blood and brain samples were taken. Antibiotic levels were determined by an agar-well diffusion method. Cerebral ceftriaxone concentrations ranged from 0.3 to 12 micrograms/g, with a mean value of 1.63 micrograms/g. These values were less than 2% of corresponding serum concentrations, but enough to inhibit 75% of bacterial strains recently isolated from brain abscesses in our unit.