Streptomyces ambofaciens was grown on a synthetic medium with glycerol (10 g litre - 1 ) and ammonium (20 mm) as carbon and nitrogen sources, respectively. Batch fermentation produced a specific production rate of spiramycin (q s p ) of 0.3 mg h - 1 g - 1 DCW. Improvement of spiramycin production was achieved by fed-batch culture. The strong influence of the initial uptake rates of glycerol (q g l y ) and ammonium (q N H + 4 ) on the specific production rate of spiramycin (q s p ) was demonstrated by conducting several fermentations with continuous feeding of glycerol and ammonium, in different ratios. The specific production rate of spiramycin was increased 10 fold. A low specific growth rate was also necessary for spiramycin biosynthesis.