A comparison of two LC methods for the separation of erythromycin is described. The first method was previously developed in this laboratory and uses an Xterra RP18 column. Acetonitrile 0.2 M K2HPO4 pH 70 - water (35:5:60) is used as a mobile phase. The second method was developed by Abbott and uses an Astec C 18 polymeric column. The mobile phase consists of acetonitrile - 0.2 M K2HPO4 pH 9.0 - water (40:6:54). This method was slightly adapted before a robustness study was performed and quantitative results were examined. Both methods allow isocratic separation (1.0 mL min(-1)) of erythromycin from all its known related substances and degradation products. The Xterra method shows better selectivity and efficiency but the Astec method shows better long term stability. Both methods show clear improvements as compared to the actual method of the European Pharmacopoeia and the United States Pharmacopeia, which is less selective and less sensitive.