This study was undertaken to further establish the nature of the low molecular weight metal-binding proteins in rat testes. In all cases, control testes were compared to livers of zinc-treated rats, which are known to contain high concentrations of metallothionein. Gel filtration of testicular and hepatic cytosol revealed a major zinc- and/or cadmium-binding protein in the low molecular weight range in both tissues. This protein could be partially purified from either source by a combination of heat treatment and sequential acetone precipitation. When such partially purified preparations were further fractionated by high performance liquid chromatography using a linear gradient of 25%-40% acetonitrile in 0.1% trifluoroacetic acid, two major forms with similar retention times were seen in each tissue. The utility of this high performance liquid chromatography system for separating isoforms of metallothionein was verified by separation of commercially available purified rabbit hepatic metallothionein into a total of five separate forms. Amino acid analysis of the two proteins derived from rat liver was consistent with the known amino acid composition of metallothionein. However, the two testicular forms separated by high performance liquid chromatography were notably different in amino acid composition from metallothionein, with a distinctly lower content of cysteine. These results indicate that the major low molecular weight cadmium/zinc-binding proteins in rat testes are not metallothioneins.