Clavulanic acid is a potent inhibitor of bacterial beta-lactamases, and ticarcillin is a potent antipseudomonal penicillin. The combination of ticarcillin disodium and clavulanate potassium provides an excellent spectrum of activity against the majority of bacterial pathogens responsible for serious infections in both normal and abnormal hosts. Eighteen courses of therapy were administered to 16 patients; 35 percent of the patients were in poor or critical condition, and all but one had severe underlying disease. Thirteen separate episodes of pneumonia were treated, of which nine were in patients with cystic fibrosis, and 11 involved Pseudomonas aeruginosa. Of the 13 cases of pneumonia, 11 showed clinical cure or improvement, whereas only three showed bacteriologic cure. Of the four nonpulmonary cases, three showed clinical improvement or cure, and one showed a bacteriologic cure. In two patients, phlebitis developed at the site of intravenous infusion. The combination of ticarcillin and clavulanic acid is safe and effective therapy for pneumonia in anatomically compromised hosts.