In vitro affinity of Cu2+ to metallothionein was stronger than Cd2+ and Zn2+. The passive incorporation of Cu into the metallothionein and the possibility of demasking of the masked Cd toxicity as metallothionein is suggested. Cu contents of Cd-exposed rat kidney metallothionein were analyzed along with Cd and Zn. The Cu in kidney metallothionine was always one of the major metals along with Zn and Cd. The Cu in Cd-exposed liver metallothionein was a minor metal.