Abstract: OBJECTIVE To set up a method for the assay of high molecular impurities in Mezlocillin Sodium for injection by TSK gel, and to disguss the internal relationships of the results with those of related compounds and polymers. METHODS The analysis was carried on a TSKgel G2000SWxl column with a mobile phase of 0.005 mol·L^-1 phosphate buffer with the pH value of 7.0. The detection wavelength was 210 nm. 204 batches of samples were determined, correlative analysis was performed to the results. RESULTS The established method could determine the high molecular impurities in Mezlocillin Sodium for injection. The correlative analysis showed that the determined impurities include the polymers and the known impurities whose molecular were higher than Mezlocillin. CONCLUSION For the polymer analysis of β-lactams, TSK gel method can not replace Sephadex G-10 method. Further studies were required to identity peaks of known high molecular impurities and those of polymers during the analysis.