Abstract： Objective To evaluate antibacterial activities of Cefoperazone-Sulbactam upon gram negative bacilli,and compare the differences in susceptibility between two different concentrations of Cefoperazone-Sulbactam combination disc.Method A total of 381 strains of commonly occurred gram negative bacilli were found from 2nd Affiliated Hospital of Zhejiang University School of Medicine,Zhejiang Provincial People's Hospital,The Third Hospital of Hangzhou and Hangzhou Hospital of Traditional Chinese Medicine respectively.Susceptibility test was conducted by K-B method using 75 μg/disc Cefoperazone plus with 75 μg/disc Sulbactam(150 disc)and 75 μg/disc Cefoperazone with 30 μg/disc Sulbactam(105 disc),respectively.Meanwhile the minimal inhibitory concentration(MIC)of Cefoperazone-Sulbactam was determined by standard agar dilution.The data were analyzed by using WHONET 5.4 and SPSS.Results Disc diffusion method was carried out to detect the antibacterial activities upon Escherichia coli,Klebsiella pneumonia,Pseudomonas aeruginosa and Acinetobacter baumannii,using 105 disc and 150 disc,respectively.The data indicated that consistency rates between these two different discs were 26.3%,79.2%,83.7%and 33%,respectively.The k-related sample test was performed by using SPSS version 10.0 and shown that the P value was less than 0.05.Upon those organisms mentioned above,the consistency rates between the antibacterial activities of 105 disc and those of agar dilution were 77.8%,89.6%,70.9%and 77%,respectively.When it was going to compare agar dilution vs.150 disc upon the susceptibility of those organisms the consistency rates were 27.3%,79.2%,61.6%and 30%,respectively.Compared with agar dilution,the error rates of those two different concentration discs revealed that the false susceptibility and false intermediate of 105 disc were higher than those of 105 disc.ConclusionsThe results of susceptibility test showed that 105 disc was more close to agar dilution than that of 150 disc.However,the 150 disc used in clinic led to increase in sensitivity of susceptibility test to organisms.