Among the plant species cultivated for human consumption, tobacco is one that accumulates cadmium to a significant degree. In order to reduce the Cd levels in tobacco leaves, we have introduced into the tobacco genome a gene encoding a mammalian metallothionein, since these low-molecular-weight cysteine-rich proteins fix Cd and other heavy metals. Here we describe the Cd accumulation characteristics observed during two years of greenhouse tests and one year of field trial of tobacco plants expressing a metallothionein gene. In all three tests, leaf Cd levels were markedly decreased. For instance, in the field trial, Cd levels in the leaf lamina tissue of the transformed line were decreased by 73% compared to controls. The decrease in leaf Cd was correlated with an increase in Cd in the roots and stems. The plants had normal growth characteristics, and the distribution of other ions was not affected by the expression of the metallothionein gene. Even though the transformation/expression vector used here can cause frequent post-transcriptional gene silencing, comparison of hemizygous and homozygous individuals of the plant line expressing the metallothionein gene gave little evidence for silencing.