Some of the relevant features of the clinical pharmacology of the tetracycline antibiotics are reviewed, and, in particular, the absorption, excretion, and disposition in the body of the four homologues that are currently available for clinical use are compared. The relatively minor differences in the chemical structure of these drugs are accompanied by significant differences in some of their physical properties and by some quantitative differences in their absorption, excretion, and distribution in the body, their activity against different bacteria, and their clinical toxicity.