Gram-negative bacilli were identified within 3 to 6 h by determining susceptibility to 18 different antibacterial agents in the Autobac I system and by applying a two-stage quadratic discriminant analysis to the susceptibility patterns. The Autobac system was compared with standard reference methods for identifying glucose nonfermenters and glucose fermenters. Intralaboratory and interlaboratory precision of the Autobac system was comparable to that of the reference methods. Sensitivity (accuracy) and specificity of the two systems were also comparable, although there were some differences with certain species. Autobac responses were considered to be equivocal (needing additional tests) if the relative probability of an accurate identification was less than 0.70. Only 5% of 2,889 strains produced such equivocal results; a similar number of strains gave low probability levels with the reference methods. When the two systems disagreed, an independent reference laboratory arbitrated, confirming 49% of the Autobac responses and 36% of the reference identifications. With equivocal responses excluded, the overall accuracy of the Autobac system was 95.3% compared with 95.9% for the reference method. The respective accuracy estimates would be 93.8% and 93.1% if all first-choice identifications were evaluated.