Soluble proteins isolated from tissues of the tomato Lycopersicon esculentum, after inoculation with Agrobacterium tumefaciens to induce tumours, have been examined by gel electrophoresis and cytochemically. Changes that occur include the suppression of host enzymes, the appearance of bacterial enzymes in the host tissues and the appearance of new enzyme bands in the affected cells. These changes are detectable within 6 hr of infection and prior to evident morphological changes, and may be explained as derepression and repression of host genes, or the expression of released bacterial genes in the host cells.