The primary structures of N-terminal 19-mer peptides, released by limited trypsin treatment of coat protein (CP) subunits in intact virions of three potato virus X (PVX) isolates, were analyzed. Twowild-type PVXstrains,Russian (Ru) andBritish (UK3),wereusedandalso theSTmutantof UK3 inwhich all 12 serine and threonine residues in the CP N-terminal segment were replaced by glycine or alanine.