Prior to the introduction of new beta-lactam antibiotics, such as ureidopenicillins and some third generation cephalosporins, the choice of antibiotics for the treatment of meningitis caused by Ps. aeruginosa was limited, and even now, this infection remains of poor prognosis. Five patients with meningitis, aged from 18 to 75 years, were treated with azlocillin. The strains of Ps. aeruginosa isolated were all sensitive to azlocillin, the zones of inhibition being greater than 20 mm in diameter with bacteriostatic concentrations. Azlocillin was administered 8-hourly by intravenous injection in doses of 30 g/day in 3 cases and 15 g/day in 2 cases. The mean duration of treatment was 33 days (range 23-50 days). In addition to azlocillin 4 patients received an aminoglycoside (tobramycin or amikacin) parenterally and, in one case, intrathecally. Four patients underwent a surgical operation on the focus of infection between the 9th and 13th days of treatment. In 3 of these 4 patients surgery was necessary to obtain apyrexia, but in all cases the CSF was already sterile when it was performed. The outcome was favourable in all 5 cases. One patient relapsed on 4 occasions due to persistent petrous bone fistula; each time, the azlocillin treatment was reinstituted and brought about clinical and bacteriological cure. The germ was eradicated in CSF on the 3rd day of treatment in 3 patients and on the 7th day in one. One patient developed transient eosinophilia and another, transient neutropenia after 30 days. It is concluded that azlocillin, associated with an aminoglycoside, is an active and effective antibiotic for the treatment of Ps. aeruginosa meningitis.