A new nucleoside antibiotic, mildiomycin, was isolated from the culture filtrate of Streptoverticillium rimofaciens B-98891 in our laboratories. It shows strong activity against powdery mildews on various plants, and remarkably low toxicity in mammals and fishes. This paper deals with the structural elucidation of mildiomycin carried out on the basis of chemical degradations and spectral evidence as shown in Chart I; Specific 13C labeling of proteins has enhanced the usefulness of 13C NMR spectroscopy as a tool for the study of these macromolecules. One highly selective method for 13C enrichment of proteins which permits their observation in an essentially native form is the methyl exchange reaction at methionyl residues. This method has now been applied in our laboratories to the basic pancreatic trypsin inhibitor (BPTI). In the course of this work we have had occasion to make spectroscopic observations on the 13C labeled protein intermediate, which possesses an enriched S-methylmethionyl residue at position 52 ( [t-13C-SMM-52]-BPTI)