A capillary electrophoresis method was developed for the pharmaceutical analysis of piperacillin, a β-lactam antibiotic, in preparations for intramuscular and intravenous injections. Disodium hydrogenphosphate and sodium tetraborate buffer pH 6.2 or 8.7 supplemented with sodium dodecyl sulfate 7 g/l and electrophoresis voltage -18 kV, seem to provide optimal conditions for piperacillin CE assay. The method was validated and good reproducibility, precision, accuracy and assay linearity in antibiotic concentrations 0.08-2.00 mg/ml for both electrolytes were observed. The detection limit for piperacillin was 0.01 mg/ml. Preliminary experiments showed the usefulness of CE for separation and determination of piperacillin and β-lactamase inhibitor co-existing in the drug Tazocin. The results obtained by CE were also compared with those obtained by liquid chromatography. Statistical analysis by Student's t-test showed no significant differences between the results obtained by the two methods.