N-acetylcysteine, carboxymethylcysteine and thiazolidincarboxylic acid salts of erythromycin-2-propionate are produced by charging erythromycin-2-propionate and the sulphur-containing acid into a reactor. The reactants are homogenised and milled under nitrogen for 1 - 2 hours at 10 - 20 DEG C. To this mixture purified water is added under nitrogen and the resultant aqueous mixture is further homogenised and milled for 1 to 2 hours at 10 - 30 DEG C under nitrogen to produce a suspension of the desired salt. The suspension is dried under vacuum to produce a high yield of the desired salt at high levels of purity. Carboxymethylcysteine, N-acetylcysteine and thiazolidin-carboxylic acid salt of erythromycin, clarithromycin, roxithromycin and azithromycin may be produced in a similar manner.