The erythromycin producer, Saccharopolyspora erythraea ER720, was genetically engineered to produce 6,12-dideoxyerythromycin A, a novel erythromycin derivative, as the major macrolide in the fermentation broth. Inspection of the biosynthetic pathway for erythromycin would suggest that production of this compound could be achieved simply through the disruption of two genes, that encoding the erythromycin C-6 hydroxylase (eryF) and that encoding the erythromycin C-12 hydroxylase (eryK). The double mutant. however, was found to produce a mixture of 6,12-dideoxyerythromycin A and the precursor, 6-deoxyerythromycin D. Complete conversion to the desired product (to the limit of detection by TLC) was achieved by inserting an additional copy of the eryG gene, encoding the erythromycin 3 "-O-methyltransferase and driven by the ermE* promoter, into the S. erythraea chromosome.