The automated Cobasbact system was adapted for direct antimicrobial susceptibility testing of positive blood culture broths and its performance compared with that of the conventional Kirby-Bauer agar disc diffusion method using 278 positive blood samples. Overall, 1746 antibiotic-organism combinations were tested. Full agreement was 86.9% and essential agreement (i.e. including minor discrepancies) was 91.8%. The system would seem to produce acceptable susceptibility results within five hours after detection of a positive blood culture broth.