Summary The conclusion by Barker (1975) that the four proteins previously thought to occur only in virus-infected tobacco leaves are probably normal constituents is based on an erroneous interpretation of the banding pattern obtained by polyacrylamide gel electrophoresis. Patterns shown by Barker for non-infected and water-injected tobacco leaves are similar to those for non-inoculated or water-inoculated plants from our laboratory. ‘New’ protein components induced by virus infection relate to bands not observed under the above-mentioned conditions. This is illustrated by a comparison of protein patterns from water-inoculated and TMV-inoculated tobacco plants, obtained under various conditions and after staining with either amido black or Coomassie blue. Preferential extraction of the new protein components at low pH further supports the view that these components do not occur in non-infected plants.