A NUMBER of terrestrial plants accumulate large quantities of metals such as zinc, manganese, nickel, cobalt and copper in their shoots(1). The largest group of these so-called ’metal hyperaccumulators’ is found in the genus Alyssum, in which nickel concentrations can reach 3% of leaf dry biomass(2,3), Apart from their intrinsic interest, plants exhibiting this trait could be of value in the decontamination of metal-polluted soils(4-6). However, the biochemical basis of the capacity for metal accumulation has not been elucidated. Here we report that exposing hyperaccumulator species of Alyssum to nickel elicits a large and proportional increase in the levels of free histidine, which is shown to be coordinated with nickel in vivo. Moreover, supplying histidine to a non-accumulating species greatly increases both its nickel tolerance and capacity for nickel transport to the shoot, indicating that enhanced production of histidine is responsible for the nickel hyperaccumulation phenotype in Alyssum.