Tetracyclines have a broad range of clinical usefulness because of their broad antimicrobial spectrum of activity. For most routine gram positive and gram negative infections, alternative agents are available, but for Chlamydiae, Rickettsiae, Brucella, and Borrelia they still remain agents of choice. To some extent, gastrointestinal intolerance and inability to use these agents in patients with renal dysfunction have been overcome by the availability of doxycycline. Phototoxicity is a problem with this agent, however. Tetracycline is still useful as a sclerogenic agent for malignant effusions, and demeclocycline often is an agent of choice in therapy of inappropriate antidiuretic hormone secretion.