220 nm filtrates of intestinal resections from 6 of 10 patients with Crohn's disease produced a cytopathic effect in WI-38 tissue-culture monolayers, whereas controls had no effect. Characterisation of the virus responsible has been completed in 3 of the 6 positive isolates and indicates that it is an R.N.A. virus, 55-60 nm in diameter, heat, ether, and acid stable, and antigenically related to Nebraska calf-diarrhoea virus. It therefore belongs to the Reoviridae family.