It has recently been demonstrated that antibodies against atrial natriuretic peptides (ANP) recognise analogues in plants and that rat ANP binds specifically to isolated plant membranes and promotes stomatal guard cell opening in a concentration dependent manner. Here we report the isolation and immunoaffinity purification of plant natriuretic peptide (PNP) from ivy (Hedera helix) with rabbit anti-alpha-ANP (1-28) (human, canine) antiserum. We also demonstrate that immunoaffinity purified plant peptide induces stomatal opening in a concentration dependent manner. This is therefore the first report of an active indigenous peptide hormone in plants. We propose that PNPs are part of a signalling system that has evolved early in evolution and is involved in the regulation of ion transport and transpiration in plants.