About 84% of the hydroxyproline residues in a cell culture of tomato (Lycopersicon esculentum x Lycopersicon peruvianum) were present in phenol-inextractable (i.e. covalently wall-bound) material. Treatment of the cells with any of three fungal elicitors (wall fragments from Phytophthora megasperma and Pythium aphanidermatum and xylanase from Aureobasidium pullulans) or with 1 mM H2O2 had little effect on the quantity of phenol-inextractable hydroxyproline per milligram of freeze-dried cells. However, each treatment induced a decrease in the content of phenol-inextractable isodityrosine (Idt) residues. Each treatment, except with the P. megasperma fragments, also induced an increase in phenol-inextractable di- (Di-Idt). The increase in Di-Idt partly accounted for the loss of Idt. We conclude that the elicitors and H2O2 acted to reinforce the existing cross-linking of cell wall (glyco)proteins by evoking oxidative coupling reactions to convert Idt to Di-Idt plus unidentified products. The promotion of cross-linking by elicitor treatment is proposed to be a defensive response that restricts the penetration of pathogens.