Abstract: In order to evaluate the clinical rationality of combined medication of kanamycin sulfate with florfenicol and to provide solid basis for further assessment of its safety indicators, the acute toxic effects of the combined medication of these two drugs on mice was investigated. The reported LD_（50） of kanamycin sulfate and florfenicol by intraperitoneal injection were 1 973.95 mg/kg and 636.68 mg/kg respectively, and the reported 95% confidence limit of kanamycin sulfate and florfenicol by intraperitoneal injection were 1 969.22-1 978.71 mg/kg and 538.5-735.1 mg/kg respectively. Based on these reported pharmacological parameters, the LD_（50） of combined medication of kanamycin sulfate with florfenicol was determined by equivalent diagram method.The results showed that in the combined medication, the LD_（50） of kanamycin sulfate by intraperitoneal injection in mice was 510.51mg/kg and the 95% confidence limit ranged from 437.52 to 595.66 mg/kg; the LD_（50） of florfenicol by intraperitoneal injection in mice was 765.60 mg/kg and the 95% confidence limit ranged from 656.15 to 893.31 mg/kg. Furthermore, the intersection of LD_（50） dose line of combined medication of kanamycin sulfate and florfenicol fell outside the upper limit of 95% confidence limit,indicating that the combined medication of these two antibiotics produced antagonism of toxic effect. These results suggest the clinical rationality and reliability of combined medication of kanamycin sulfate with florfenicol.