When the agar dilution method is used for performing antimicrobic susceptibility tests, Mueller-Hinton agar plates may be prepared with varying concentrations of the drugs to be studied. With most antimicrobics, the plates can be prepared in advance and stored in the refrigerator for as long as four weeks. The present study documents the instability of minocycline when stored under refrigeration in agar plates. Doxycycline and tetracycline also lost activity but at a slower rate. Degradative changes occurred most rapidly at the lower concentrations; at 16 μg/ml or greater, the drugs were relatively stable. When diluted in Mueller-Hinton broth and stored at −60°C in microdilution trays, the three tetracyclines could be held up to six weeks with no loss of bioactivity. Concentrated stock solutions were kept at −60°C for six months with no loss of potency.