A simple, sensitive, selective and robust isocratic LC method is described for the analysis of erythromycin on XTerra RP18 The main component, erythromycin A is separated from all known related substances and degradation products. Several unknown impurities are also separated. Acetonitrile - 0.2 M K2HPO4 pH 7.0 - water,(35:5:60, v/v) was used as a mobile phase at 1.0 mL min(-1). UV detection was at 215 nm. The robustness of the method was evaluated by a full-factorial experimental design.