In treating severe respiratory infections due to Pseudomonas aeruginosa and in patients with cystic fibrosis, new anti-pseudomonal beta-lactams constitute a good alternative to the traditional aminoglycoside antibiotic therapy. Among several criteria for the choice of an antibiotic, estimation of intrabronchial levels is to be taken into account. The authors report the results of the study of a new semi-synthetic penicillin, apalcillin, and two cephalosporins (cefsulodin, ceftazidime), active in vitro against Pseudomonas; their penetration into respiratory secretions was evaluated in 48 patients, intubated, tracheostomized, or undergoing fiberoptic bronchoscopy. Antibiotic concentrations were measured by the microbiological procedure. Bronchial penetration of apalcillin was early and noticeable, reaching a bronchial peak representing 20% of the simultaneous seric concentration. The local levels of cefsulodin and of ceftazidime did not differ from those achieved with cephalosporins studied previously; the bronchial peak reached 5,6 micrograms/ml for both drugs 2 hours after administration; this bronchial level corresponded to 15 to 20% of the serum concentration. The clinical response could not be evaluated due to the severe underlying pathology which compromised the outcome of the disease in the patients of this study. However, at least for apalcillin, a significant correlation was noted between bronchial levels, susceptibility of bacteria isolated in patients, and eradication of susceptible organisms.