When potato virus X (PVX) is exposed to enzymically oxidizing chlorogenic acid, and re-isolated, some of its properties are changed. Its u.v. spectrum is modified and centrifuged pellets are coloured: its u.v. fluorescence is diminished, a longer-wave fluorescence is introduced: it produces less colour with 2,4,6-trinitrobenzene sulphonic acid, and moves faster during electrophoresis. There is, however, little or no loss of infectivity in the course of the reaction. The results suggest that PVX combines with chlorogenoquinone to produce a modified but infective virus (PVX-Q) and it is thought that this reaction may occur naturally.