50% inhibitory concentration (IC50) and minimum inhibitory concentration (MIC) of ceftazidime, cefsulodin, cefotaxime, moxalactam, azlocillin, carbenicillin, netilmicin and tobramycin against 90 strains of Pseudomonas aeruginosa isolated from cystic fibrosis (CF) patients were determined by means of the agar plate dilution method. Ceftazidime was most active of the antibiotics in vitro; the geometric mean of IC50 was 0.2 and of MIC 0.4 micrograms/ml. 11 CF patients suffering from P. aeruginosa infection were treated with 14 day courses of ceftazidime (100 mg/kg/24 h) and tobramycin (10 mg/kg/24 h). P. aeruginosa was only temporarily eradicated in one of the patients, but a significant improvement of respiratory function and a significant fall in white blood cell count was recorded in the patients during chemotherapy. There was no development of resistant strains against ceftazidime during treatment and no side-effects were observed. Ceftazidime is a promising new antimicrobial agent with high in vitro activity which deserves further in vivo evaluation in CF patients.