Tobacco leaves pretreated with low levels of an Erwinia chrysanthemi pectate lyase isozyme did not express the hypersensitive response which normally follows inoculation with Pseudomonas syringae pv. pisi . The enzyme was obtained from the periplasmic shock fluids of an Escherichia coli strain containing a cloned pectate lyase gene ( pelC ) from E. chrysanthemi . Crude shock fluids or homogenous pectate lyase purified from the fluids were effective in preventing the hypersensitive response in tobacco leaves; shock fluids from E. coli lacking the pelC gene and buffer treatments were ineffective. Prevention of the hypersensitive response by pectate lyase was light-dependent and required an induction period of approximately 1·5 h prior to challenge with P. syringae pv. pisi . Pretreatment of tobacco leaves with a purified isozyme preparation containing 0·03 units (umol product min −1 at pH 6·0) of activity per millilitre and less than 3 ng protein ml −1 caused suppression of the multiplication of both compatible P. syringae pv. labaci ) and incompatible ( P. syringae pvs. pisi and syringae ) bacteria. Pectate lyase pretreatment also inhibited the production of disease symptoms by P. syringae pv. labaci . Heat-stable products released by pectate lyase from isolated tobacco cell walls were similarly able to suppress the hypersensitive response in tobacco leaves, suggesting that the protection phenomenon may be mediated by galacturonic acid-containing oligosaccharides.