Susceptibility test results from 100 clinical isolates, using the AMS, MS-2, Autobac MTS, Micro-Media system, and Sensititre, were compared with results from the proposed National Committee for Clinical Laboratory Standards reference microdilution method for minimum inhibitory concentrations and with Bauer-Kirby results. Isolates were tested concurrently by each method on consecutive days to obtain duplicate results. The data were computer analyzed, using National Committee for Clinical Laboratory Standards guidelines for break point interpretation. Analysis was centered on drug-organism combinations and not on overall percent correlation. Data were analyzed for comparability to the reference methods and for reproducibility within each system. Commercial system results were very reproducible. Results from 4- to 8-h tests (AMS, MS-2, MTS) gave more very major discrepancies when compared with either reference method than did results from 15- to 18-h systems (Micro-Media, Sensititre).