The tetracyclines are a group of closely related bacteriostatic antibiotics with similar antibacterial spectra. The tetracyclines are variably absorbed after oral administration and penetrate into most tissues and body fluids, with the highest levels in kidney and liver. All tetracyclines are excreted in urine and faeces, either as the parent molecule or as microbiologically inactive forms. The inactive 4-epimers (33) are thought to be artefacts produced by chemical rather than biological processes and can form spontaneously during sample preparation (32). Tetracyclines are used extensively in veterinary medicine and their use in food producing animals could result in potentially harmful concentrations in tissue, organs and milk. The potential risk is reduced by withdrawal of the drug for a fixed period before slaughter, although residual levels may remain.