A rapid, highly sensitive and reproducible high-performance capillary electrophoresis (HPCE) method (electrokinetic chromatography with sodium dodecyl sulfate) is described for the determination of the polysaccharide from the uropathogenic Escherichia coli K4 bacteria (05:K4:H4) and its defructosylated product. The two polyanions, K4 and defructosylated K4, are separated and readily determined within 30 min on an uncoated fused-silica capillary using normal polarity at 20 W and detection at 200 nm. A linear relationship was found for the two polysaccharides over a wide range of concentrations, from approximately 30 ng (0.5 mug/muL) to 210 ng (3.5 mug/muL). The described method was used to evaluate the defructosylation process of K4 under drastic acid conditions.