A combination of native electrophoresis and immunodetection (Western blot) was used for the characterization of nucleoprotein particles of the potyvirus Plum pox virus (PPV). Virus particles were electrophoresed directly from plant extracts in agarose or mixed acrylamide-agarose gels under native conditions, blotted on nitrocellulose membranes, and characterized with the aid of a coat protein-specific antibody. Using this combined methodology, called NEWeB (native electrophoresis and Western blotting), we could show that a population of particles that differ in their electrophoretic mobility can be detected in extracts of Nicotiana benthamiana, that two different strains of PPV can be distinguished in double infections of the same plant and that virus particles from leaves contain detectable levels of helper component proteinase molecules. The potential of the NEWeB method for the study of structure and function of virus particles and similar nucleoprotein complexes in single and mixed infections is discussed.