Bronchial mucosal biopsies were obtained during fibreoptic bronchoscopy in 12 patients receiving a new semisynthetic penicillin, piperacillin. The piperacillin levels estimated in bronchial mucosa exceeded those required to eradicate organisms associated with acute bronchitis, Haemophilus influenzae and Streptococcus pneumoniae, and compared favourably with those required for activity against a wide variety of anaerobic and Gram-negative organisms including Pseudomonas aeruginosa. Sputum and serum piperacillin levels were obtained from eight patients with bronchial disease receiving a five to seven day course (8 to 16 g/day). Sputum/serum level ratios were constant for the two dosages (10.7% for 8 g/day; 14.3% for 16 g/day) suggesting a diffusion transfer process, although the presence of pus in the sputum appeared to facilitate penetration. Seven patients achieved sputum levels exceeding those required for activity against Haemophilus influenzae and Streptococcus influenzae, and four for Pseudomonas aeruginosa. This study provides pharmacolinetic support of the use of piperacillin in bronchopulmonary infection.