1. A low molecular weight (7,700) Zn- and Cu-containing protein was isolated from the livers of Zn-injected rainbow trout by gel filtration and ion exchange chromatography. Purity of the isolated protein was assessed by native and denaturing polyacrylamide gel electrophoresis and isoelectric focusing. 2. The purified protein was positively identified as a metallothionein on the basis of its molecular size, high metal content (3.6 g atoms Zn and 2.6 g atoms Cu per mole; 5.2% metal), heat stability, u.v. absorption spectrum, charge and amino acid composition (25% cysteine, no histidine and tryptophan, and trace tyrosine and phenylalanine). 3. The relatively high Cu content of this protein was unexpected and may be attributed to the presence of high levels of Cu, as Cu-thionein, in the livers of non-injected fish. 4. The comparative differences in the metal content of hepatic MT in trout and other animals are discussed.