Although a variety of biosynthetic studies have been focused on macrolide antibiotics1~3), the biosynthetic processes, especially the final steps in the biosynthesis of the 14-membered-ring macrolide antibiotic, oleandomycin (1) have not yet been well elucidated. Recently, an Abbott group isolated (8R)-8,8a-deoxyoleandolide (3) from a fermentation broth of an erythromycin-producing strain, and suggested the aglycone 3 to be a biosynthetic intermediate of oleandomycin4). We have chemically synthesized the aglycone 3 and the related aglycones including the intact aglycone, oleandolide (5) for chemical simulations of the biosynthesis5'6) and the total synthesis of oleandomycin (1)7'8). The syntheses of various aglycones enabled us to study their incorporation by using blocked mutants of an oleandomycin-producing organism, Streptomyces antibioticus9). Herein, a plausible scheme for the final steps in oleandomycin biosynthesis will be proposed by the incorporation studies.