A method of making a stable liquid composition containing a harpin protein or polypeptide, an aqueous carrier, an effective amount of a biocidal agent, and optionally, an effective amount of one or both of a protease inhibitor and a non-ionic surfactant is disclosed. The composition retains harpin activity for at least about 72 hours. Also disclosed is a method of inducing a plant response by applying a composition to a plant or a plant seed.