The MICs of 10 antimicrobial agents were determined for 1,000 clinical aerobic and facultatively anaerobic gram-negative bacilli by the 6-h AutoMicrobic system Gram-Negative General Susceptibility-Plus Card (GSC+) (Vitek Systems, Inc., Hazelwood, Mo.) and by an 18-h reference agar dilution method. Results obtained by both systems were evaluated for twofold dilution discrepancies and for interpretive category discrepancies. MICs that differed by more than one twofold dilution between the two test systems were considered to be discrepant. The overall twofold dilution agreement of the GSC+ MICs to agar dilution MICs was 94.0%. The agreement for each drug tested ranged from 91.7% (ampicillin and chloramphenicol) to 96.5% (carbenicillin). Interpretive categories were considered in agreement when the susceptibility interpretations obtained by the two systems were both very susceptible, moderately susceptible, or resistant. The overall interpretive category agreement was 80.5%. This agreement increased to 95.8% when all minor errors (moderately susceptible, one system; very susceptible, the other system) that would not affect therapy were omitted. The AutoMicrobic system GSC+ provides rapid and accurate susceptibility test results for clinical aerobic and facultatively anaerobic gram-negative bacilli.