We have investigated the effectiveness of seven new beta-lactam antibiotics, azlocillin, piperacillin, ceftazidime, cefsulodin, cefoperazone, latamoxef (moxalactam), and cefotaxime, against acute pulmonary exacerbations caused by Pseudomonas aeruginosa in cystic fibrosis. Three hundred and fifty-five strains of Ps aeruginosa isolated from 310 sputum cultures (190 cystic fibrosis patients) were tested for susceptibility to the drugs by determination of minimal inhibitory concentrations (MIC). The highest activity was shown by ceftazidime (6% resistant strains) followed by cefsulodin and piperacillin (15 and 16% resistant strains); very low activity was found for cefotaxime and latamoxef (moxalactam). Ceftazidime was the most active drug against 32 pseudomonas isolates that were resistant to both carbenicillin and aminoglycosides (78% susceptible). A randomized, double-blind trial of azlocillin, piperacillin, ceftazidime, cefsulodin or cefoperazone was performed in 111 cystic fibrosis patients with predominant and susceptible pseudomonas in their sputum. Results were evaluated by a clinical, radiological and bacteriological scoring system: the best results were obtained with ceftazidime, followed by cefsulodin and piperacillin. However, pseudomonas was eradicated in only 22 (23%) of the cases with the most active drugs and persisted or reappeared in all the cases 1 to 3 months later. Ceftazidime always eradicated Staph. aureus and Haemophilus influenzae associated with pseudomonas. Similar eradication occurred nearly always with cefsulodin but rarely with the other drugs. No serious drug reaction occurred but a later fever and rash with piperacillin, transient diarrhoea with cefoperazone, vomiting with cefsulodin, and very frequent eosinophilia with ceftazidime should be mentioned. These five drugs offer, in varying degree, alternatives to traditional anti pseudomonas antibiotics in cystic fibrosis pulmonary infections, but they should be used only against well-proven resistant strains. Ceftazidime is best and cefotaxime and latamoxef (moxalactam) least useful.