In normal human subjects multiple dose trial with an initial dose of 400 mg doxycycline and subsequent doses of 200 mg every 24 hours gave average minimum serum concentrations of between 1 and 2 yg doxycyclinelml serum. The average urine concentrations varied between 100 and 200 yg doxycycline/rnl urine, and 42 per cent of the drug administered was excreted in the urine in an active form. The average renal clearance of doxycycline determined over 96 hours was 28 ml/min. In a single dose trial with 200 mg doxycycline, an 8-hour renal clearance of doxycycline was found to be 13 per cent of the creatinine clearance, and the serum half-life period was found to be 11.6 hours. Microbiological assay was used for all determinations of doxycycline in the serum and urine. The in vitro sensitivity of 211 urinary tract pathogens to doxycycline and oxytetracycline was investigated in order to determine the minimum inhibitory concentration (MIC). The strains, which originated from patients with chronic urinary tract infections, were as a whole only slightly sensitive to the two tetracyclines. Compared with oxytetracycline, doxycycline, assessed at the MIC-level, showed a higher bacteriostatic effect in vitro, especially against gram-positive strains. doxycycline: absorption, distribution et excrCtion d'un nouvel antibiotique a large spectre chez I'homme. Schweiz. med. Wschr. 1967,97, 915-923. Kirschbaum, A., J. Kramer & M. A. Garth: Uniform preparation of microbial suspensions for antibiotic assays. Antibiot. and Chemother. 1962, 12, 545-550. Kunin, G . M., A. C. Dornbush & M. W. Finland: The distribution and excretion of four tetracycline analogues in normal males.