Piperacillin, a new semisynthetic penicillin, was evaluated for efficacy and safety in 26 patients, most of whom had pneumonia. Included were four patients with gram-negative meningitis in whom the penetration of piperacillin into cerebrospinal fluid was determined. Cure was achieved in 11 of 17 patients with pneumonia; another 4 were improved. One relapse and one failure occurred among nine patients with gram-negative pneumonia. Cure or improvement occurred in seven of nine patients with gram-negative infection in various extrapulmonary sites. Piperacillin given by continuous infusion in a dosage ranging from 324 to 436 mg/kg of body weight per day to four patients with meningitis resulted in a mean cerebrospinal fluid level of 23 micrograms/ml at 24 h; the mean penetration of piperacillin into the cerebrospinal fluid was 32% at this interval. Levels of piperacillin in cerebrospinal fluid collected later during the course of therapy were also adequate. Adverse effects were noted in six patients, but only one episode of granulocytopenia was serious. Emergence of resistance to piperacillin did not occur, and only one superinfection was noted. Piperacillin appeared to be efficacious in the treatment of pneumonia. It penetrated well into the cerebrospinal fluid of patients with meningitis and may be useful for treatment of selected gram-negative infections in extrapulmonary sites.