Effects of two ammonium salts on the erythromycin production from Saccharopolyspora erythraea were investigated. Higher initial concentrations of ammonium sulfate reduced the final erythromycin production yield of a mutant strain by 77% to the benefit of a biomass accretion. High concentration of ammonium nitrate favoured erythromycin production yield (double increase). A rapid pellet formation may have contributed to the early onset of antibiotic production.