A simple and selective method for the determination of erythromycin and its stearate and succinate esters in their pure forms and in pharmaceutical formulations is described. The procedure is based on the formation of a blue-coloured (lambda max = 633 nm) complex with gentiana violet in alkaline medium. Different variables affecting the colour development were studied and optimized. The method was used to determine 2.5-25 micrograms ml-1 of erythromycin in the final measured solution. The simplicity of the method permits rapid analysis and it is thus suitable for routine control. The method is highly specific for the determination of stearate and succinate esters in pharmaceutical formulations. The reliability of the method was established by parallel determinations against the official British Pharmacopoeial method.