We have investigated the use of the new cephalosporin ceftazidime for the treatment of pseudomonas infection in cystic fibrosis, using 100 to 240 mg/kg intravenously daily. The clinical and microbiological results of 18 courses of therapy, lasting from 1 to 4 weeks have been satisfactory, particularly since the patients had previously proved refractory to treatment with most other appropriate antibiotics. However, in common with other anti-pseudomonal antibiotics, a first course of ceftazidime proved the most successful, subsequent courses being less effective. Three patients died because of chronic respiratory infection. In cystic fibrosis pseudomonal infections rapidly relapse after therapy and therefore we have been examining the interactions of antibiotics with relevant host defence mechanisms. In one patient who received a 4 g infusion of ceftazidime there was enhancement of neutrophil phagocytosis-triggered chemiluminescence. We conclude that, used in high doses, ceftazidime is a useful addition to the available antibiotics for therapy of pseudomonas infection in cystic fibrosis.