The QUANTUM II system (Abbott Diagnostics, Irving, Tex.) was evaluated with 65 species of gram-negative bacilli from various culture collections at the Centers for Disease Control. The QUANTUM II system accurately identified 92.5% of 335 isolates tested, as follows: 92.6% of 258 members of the family Enterobacteriaceae, 92.7% of 55 nonfermenters, and 91% of 22 oxidase-positive fermenters. These results were obtained by using the additional biochemical and serologic tests recommended by the manufacturers of the QUANTUM II system. The 25 misidentified cultures generally belonged to newly recognized genera, atypical strains, or slower-growing strains of more widely known genera. The system identified the most commonly encountered organisms at an accuracy of greater than or equal to 95%. The system is efficient, accurate, and rapid.