The present invention relates to the isolation and identification of novel genes of the biosynthetic pathway for spiramycins and to novel polypeptides involved in this biosynthesis. The invention also relates to a method for producing these polypeptides. It also relates to the use of these genes for the purpose of increasing the levels of production and the purity of the spiramycin produced. The invention relates in particular to a microorganism which produces spiramycin I but which does not produce spiramycin II and III, and to the use of such a microorganism. The invention also relates to the use of the genes of the biosynthetic pathway for spiramycins for constructing mutants which can lead to the synthesis of novel antibiotics or to derived forms of spiramycins. The invention also relates to the molecules produced through the expression of these genes and to pharmacologically active compositions of a molecule produced through the expression of such genes.