The protein profile on media favouring or reducing spiramycin biosynthesis in S. ambofaciens, was examined using SDS-polyacrylamide gradient gel electrophoresis over a time course of 144 h. Changes in the pattern of protein synthesis were observed which varied according to the nutrient medium conditions and appeared to suggest the presence of protein subsets specific to the production phase, which were probably involved in the switch to secondary metabolism and the onset of antibiotic synthesis. Excess of ammonium, glycerol or phosphate provoked the persistence of a protein band during the whole culture period, which was detected only during growth phase in the control producer culture. In addition, the nutrient excess caused the suppression of spiramycin production and the absence of two protein bands which appeared only in production phase in the control producer culture.