The present invention relates to a method of making a stable liquid composition containing a harpin protein or polypeptide. Also disclosed is a composition comprising an aqueous carrier, a harpin protein or polypeptide, 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. The composition retains harpin activity for at least about 72 hours. The present invention also relates to a method of inducing a plant response by applying to a plant or a plant seed the composition of the present invention.