Kinetic analysis of minocycline concentrations in plasma and urine resulted in the following findings: In normal subjects the biological half-life is about 17 hours after the first dose and 21 hours after repeated administration. The renal drug clearance is only about 8% of the over-all plasma clearance which is independent of renal function with a mean value of 47 ml/min. The fraction of the absorbed dose eliminated unchanged in the urine is only 9–19%. As a consequence the elimination rate of the drug is practically independent of renal function and decreases only 9–19% in anuric patients. The renal drug clearance depends linearly on renal function. The gastro-intestinal bio-availability of minocycline from the coated tablet preparation is incomplete. The cumulative behaviour of the drug corresponds to the kinetic parameters determined after repeated administration. It is suggested that the usual dosage regimen should be used in patients with renal disease.