An improved high-performance liquid chromatographic method for analysis of erythromycin is described. The separation can be performed under mild conditions of pH (6.5) and temperature (35 degrees C) on C8 and C18 silica-based reversed-phase materials of different origins. The mobile phase, with a flow-rate of 1.5 ml/min, contained various amounts of acetonitrile (25-40%, v/v), 5% (v/v) 0.2 M ammonium phosphate buffer pH 6.5, 20% (v/v) 0.2 M tetramethylammonium phosphate and water. UV detection at 215 nm allows quantitation of erythromycins A, B and C, N-demethylerythromycin A, erythromycin A enol ether and anhydroerythromycin A. The column history plays a major role, older columns often giving better separations.