A sensitive radioimmunoassay for rat liver metallothionein-I has been developed using avid and high-titre antibodies obtained from sheep that were immunized with a conjugate of metallothionein and rabbit immunoglobulin G. The assay was specific for metallothionein-I, and did not depend on the particular metal bound to the protein. There was no significant cross-reaction with rat liver metallothionein-II. The use of the assay to measure metallothionein concentrations in rat plasma and kidneys is described.