A column-switching technique is described for LC of erythromycin. The method allows, in about 1 h, the separation of erythromycin A from all its known potential impurities, except erythromycin D, which is a minor impurity. The switching technique combines two columns (7.5 cm x 4.6 mm and 25.0 cm x 4.6 mm) both packed with RSil C 18 LL 10 microns. The mobile phase is acetonitrile-tetrabutylammonium sulphate (0.2 M, pH 6.0)-ammonium phosphate buffer (0.2 M, pH 6.0)-water (24:5:5:66, v/v/v/v). Temperature was 35 degrees C, flow rate was 1.5 ml min-1, detection was by UV at 210 nm. Results for a number of commercial samples of various origin are reported.