Tobacco (cv. Xanthi nn) plants were watered with allopurinol [4-hydroxypyrazolo (3,4-d) pyrimidine, HPP], a xanthine oxidoreductase (XOR) inhibitor, to investigate its effects on infection by Tobacco mosaic virus engineered to express the green fluorescent protein (TMV.GFP). TMV.GFP infection was monitored by examination of inoculated leaves under UV light, by confocal scanning laser microscopy and by epifluorescence microscopy. Susceptibility to TMV.GFP was enhanced in HPP-treated plants. This was seen as a statistically significant increase in numbers of infection sites per leaf and in the number of infected cells per infection site. Two hypotheses are discussed to explain the enhanced susceptibility. The inhibition exerted by HPP against XOR activity could provoke either (i) an increased adenine and guanine nucleotide pool, which could facilitate viral RNA synthesis or (ii) it could cause changes in IAA/auxin levels, which has been proposed to influence TMV susceptibility in tobacco.