The isolation of a homologous series of five novel heavy metal complexing peptides from plant cell suspension cultures is reported. Their structures were established as (gamma-Glu-Cys)n Gly, n = 3-7. These peptides appear upon induction with heavy metals and represent the principal metal binding activity in induced cells. The name phytochelatin (PC) is proposed for this new class of natural products in higher plants. The hitherto postulated metallothioneins do not exist in plants but their function of chelating heavy metals via metal-thiolate coordination is fulfilled by the simpler phytochelatins. They are not primary gene products because of their repetition of gamma-glutamyl linkages.