A new automated method measuring the antibiotic susceptibility and the minimal inhibitory concentration (MIC) for bacteria contaminated by drug-resistant types is proposed. The growth curves of 10 drug channels and one reference channel are monitored continuously by the turbidity of the culture broth. From the analysis of the bacterial growth curves, new growth rate parameters D and G are obtained. With these parameters, MIC are calculated and displayed automatically on the monitor TV every one hour after inoculation. The profile of G and D against the concentration of drug and the time course of MIC can give information of the existence of drug-resistant bacteria. The MIC obtained by long time measuring is evaluated from the time course of MIC.