Inhibitory activity of minocycline was determined using freshly-isolated microorganisms responsible for clinical infections. The activity of minocycline was compared to that of doxycycline and tetracycline. A concentration of 3μg/ml of minocycline, well within the levels obtainable in body fluids after administration of usual doses, was inhibitory against 97% of the strains of staphylococci. This concentration was effective on more than 90% of E. coli, Proteus mirabilis, Klebsiella-Enterobacter and Pseudomonas aeruginosa, provided that the pH of the medium had been adjusted to the pH 5 level. Increase of the size of the inoculum decreased the effectiveness of minocycline, doxycycline, and tetracycline in similar proportions. The critical diameter of the zone size for distinguishing between strains sensitive or resistant to minocycline was determined as a 16-mm diameter. Since minocycline is protein bound and its effect is pH dependant, the clinical interpretation of this in vitro data must be done with caution.