The BIOMIC System (Giles Scientific, New York, N.Y.) includes software and a video-assisted plate reader that functions with a personal computer to automate, speed read, and interpret standard antibiotic disk diffusion test plates. The video reader helps standardize endpoints, speeds quantitative measurements by 40 to 90%, and reduces fatigue and transcription and interpretation errors (H. Wei-Fang, Am. Clin. Lab. 13:28-29, 1994). Organisms tested were isolated from patient specimens collected at Primary Children's Medical Center and included rapidly growing gram-positive and gram-negative strains that fulfill the National Committee for Clinical Laboratory Standards guidelines for disk diffusion susceptibility testing. A comparison of the plate reader-determined zones and visually measured zones for 3,339 organism-antimicrobial agent combinations was performed. The results demonstrated 0.1% (4 of 3,339) false-susceptible reads and 0.2% (6 of 3,339) false-resistant reads by the video reader compared with visual reads. Minor discrepancies (4.7% [156 of 3339]), resulting in category interpretation changes of intermediate to resistant or susceptible or changes of resistant or susceptible to intermediate, were also encountered. Of the discrepant results, 80.8% (139 of 172) resulted from a 3-mm or less zone diameter difference between the two different techniques. We conclude that the video-assisted plate reader is a reliable system for determining interpretative categories from zone diameters of standard antibiotic disk diffusion test plates.