Rapidly growing tomato (Lycopersicon esculentum) cell suspension cultures contain transiently high levels of cell surface, salt-elutable, monomeric precursors to the covalently cross-linked extensin network of the primary cell wall. Thus, we purified a highly soluble monomeric extensin substrate from rapidly growing cells, and devised a soluble in vitro cross-linking assay based on Superose-6 fast protein liquid chromatography separation, which resolved extensin monomers from the newly formed oligomers within 25 minutes. Salt elution of slowly growing (early stationary phase) cells yielded little or no extensin monomers but did give a highly active enzymic preparation that specifically cross-linked extensin monomers in the presence of hydrogen peroxide, judging from: (a) a decrease in the extensin monomer peak on fast protein liquid chromatography gel filtration, (b) appearance of oligomeric peaks, and (c) direct electron microscopical observation of the cross-linked oligomers. The cross-linking reaction had a broad pH optimum between 5.5 and 6.5. An approach to substrate saturation of the enzyme required extensin monomer concentrations of 20 to 40 milligrams per milliliter. Preincubation with catalase completely inhibited the cross-linking reaction, which was highly dependent on hydrogen peroxide and optimal at 15 to 50 micromolar. We therefore identified the cross-linking activity as extensin peroxidase.