The biosynthetic relationship of the nanaomycins produced by Streptomyces rosa var. notoensis OS-3966 was studied by means of a bioconversion method using the antibiotic cerulenin, a specific inhibitor of fatty acid and polyketide biosyntheses. Nanaomycin D was considered to be the first component produced from the hypothetical intermediate "polyketide". It is proposed that the biosynthesis sequence for the nanaomycin is: nanaomycin D leads to nanaomycin A leads to nanaomycin E leads to nanaomycin B. Nanaomycin B can be converted to nanaomycin A by non-enzymatic dehydration; however, nanaomycin A is rapidly bioconverted to nanaomycin E, which is the major component synthesized by the nanaomycin-producing strain.