Minocycline concentrations in serum, saliva, sputum, pleural exudate and lung extracts after a single dose of 0.2 g (orally or intravenously) were measured on 32 patients with bronchial or lung disease. On the first day of treatment, concentrations in purulent sputum were five to ten times higher than in mucous sputum and saliva, after two hours they were one third, after three hours half the serum concentration (0.7 and 1.0 microgram/ml, respectively). After six hours the concentration was the same (1.6 microgram/ml). On the third day of treatment (after 0.2 g every 24 hours) concentrations in purulent sputum were higher than on the first day by 0.8 microgram/ml (after two hours) and by 0.95 microgram/ml (after three hours). After one-hour i.v. infusion of 0.2 g minocycline concentrations in mucous sputum and saliva rose more quickly on the first day than after oral administration. On the third day of treatment (after 0.1 g orally every 12 hours) pleural exudate level was almost as high as serum level. Minocycline concentration in lung extract on the third day of treatment--four, five and ten hours after the last dose--was 0.4 and 0.8 microgram/g, respectively (while serum concentration at the same time was 1.0-1.5 microgram/ml).