Abstract The glucose analogue N-methyl-1-deoxynojirimycin was found to be a specific inhibitor of the trimming of the outermost glucose residue of the N-linked precursor-oligosaccharide Glc 3Man 9GlcNAc 2, and therefore of oligosaccharide processing, in fowl plague virus-infected chicken-embryo cells. The fowl plague virus glycoproteins in N-methyl-1-deoxynojirimycintreated cells contain oligosaccharides of the composition Glc 3Man x GlcNAc 2 ( x = 7, 8, and 9). Inhibition of trimming of the outermost glucose residues does not prevent release of infectious virus with oligosaccharides of the composition Glc 3Man 7(GlcNAc) 2. On the other hand inhibition of the trimming of the innermost glucose residue does inhibit release of infectious virus (Datema, R., Romero, P. A., Legler, G., and Schwarz, R. T. Proc. Nat. Acad Sci. USA 79, 6787–6791 (1982)).