Substantial soluble glutathione S-transferase activity and millimolar reduced glutathione (GSH) are present in most tissues of both teleosts and elasmobranchs. The hepatic enzymes of fish conjugate a range of electrophilic substrates with GSH, although their specificities are less broad than those of the transferases in rodent liver. There is no good evidence that fish transferases have ligandin-like activity or a 'suicide' function. All fish livers tested have several transferase isoenzymes. They are dimers of subunits whose Mrs are about 25 kDa and which may have different catalytic properties. In some species transferase activity is induced by agents such as phenols or 3-methylcholanthrene. Glutathione S-transferases are important detoxication enzymes in fish.