Abstract: Based on the absorption behavior of the ternary complex formed among lomefloxacin,alizarin and lanthanum in the ultra-violet band with its absorption maxima at 286 nm,an UV-spectrophotometric method for the determination of lomefloxacin was proposed.Linear relationship between the absorbance and concentration of lemofloxacin was found in the range of 0.7-12 mg·L-1,with molar absorptivity of 4.2×104L·mol-1·cm-1.As shown by experimental results,the addition of reagents should be in the order of adding firstly alizarin solution to the sample solution containing lomefloxacin,and then adding the La(Ⅲ) solution.The mole ratio of the ternary complex was found to be lomefloxacin∶alizarin∶La(Ⅲ) 1 to 1 to 2.Chloroform was used as solvent to remove excess of alizarin by extraction.The ternary complex was stable within 2 h.Temperatures within the range of 20-70 ℃ were tolerable for the complexing reaction.In the analysis of a sample of lomefloxacin capsules,the results obtained by this method were found in consistency with the labelled value and also with the results obtained by the method given in literature<参考文献原文>,Average values of RSD′s was 0.95% and of recovery was 101.1%.