A group of 41 patients, all admitted to hospital because of acute purulent exacerbations of chronic respiratory disease, were treated with either doxycycline or minocycline in a double-blind randomized study. Drug dosage was one 100 mg capsule twice daily for seven days. Bacteriological and clinical assessment before and immediately after treatment showed no significant differences between the doxycycline and the minocycline groups, nor did further evaluation after seven days follow-up. Pharmacokinetic studies showed that the Cmax and 0-11 h AUC values in blood were higher for doxycycline, whereas the sputum Cmax was, on average, higher for minocycline because of the greater penetration of the latter. The MIC values for the two antibiotics differed slightly, usually, but not always, in favour of minocycline. Problems were experienced with both agents in the eradication of Haemophilus influenzae. The net clinical results with the two drugs were identical.