A commercially available through-the-plate reading photometer (Titertek Multiskan, Flow Laboratories, McLean, Va.) was used to determine microdilution minimal inhibition concentrations (MICs) for 190 clinical isolates of bacteria (115 gram-negative rods and 75 gram-positive cocci). The performance of the instrument was evaluated by comparing its MIC readings to those that were visually determined. The MICs determined by the instrument were identical to those determined visually for 94% of the gram-negative rods and 89% of the gram-positive cocci tested. Of the MICs determined by the instrument, 99% was within one dilution of the MICs determined visually.