From the fermentation broth of Streptomyces tsukubaensis 9993 an isomer 2a of the immunosuppressant FK-506 1a was isolated as a minor metabolite. From NMR-and FAB-MS data follows that in 2a the size of the macrocycle is reduced by two carbon atoms due to a shift of the lactone-acyl from 0–26 to 0–24. The compound was synthesized from FK-506 and proved to be identical with the natural product 2a.