The Cobasbact is an automated system for use in the microbiological laboratory which is capable of performing antimicrobial susceptibility tests within five hours. The performance of the Cobasbact system was evaluated using 2000 non-fastidious, facultative and aerobic clinical bacterial isolates. The standard Kirby-Bauer disk diffusion test served as reference method. For gram-positive isolates, the rate of overall full and essential agreement between Cobasbact and reference antibiograms was 90% and 95.5% respectively, for Enterobacteriaceae 91% and 95.5% and for Pseudomonas spp. 90% and 96%. Reproducibility studies yielded an essential agreement rate of 98%. On the basis of this preliminary evaluation it would seem that the Cobasbact system can rapidly produce reasonably accurate and reproducible results of antimicrobial susceptibility tests for gram-positive cocci, Enterobacteriaceae and Pseudomonas spp.