Cytochemical studies at optical microscopic level were made to determine the chemical nature of intranuclear inclusions with fibrous aspect which were induced by the celery yellow mosaic virus (CYMV) in most of its hosts. The cytochemical tests were carried on fresh as well on fixed foliar material, fixation being in glutaric aldeid, Carnoy 3:1 or Bouin. The following reactions were tried: ferric haematoxylin (control); Sudan IV and Nile blue (for lipids); iodine ioduret (for starch); Feulgen, azur B and methyl green-pyronin (for nucleic acids); ninhydrin-Schiff and fast-green, the latter in acid and in alcaline solution (for protein). The tests with methyl green-pyronin, azur B and ninhydrin-Schiff were combined with enzimatic digestion with RNase or pepsin. The results suggest that, within the sensibility of the tests, the inclusion contains protein but does not contain starch, lipids or nucleic acids. This permit to suppose, therefore, such inclusions not to be formed of virus particles.