Five forms of glutathione transferase (GST) were resolved from the cytosol of adult common toad (Bufo bufo) liver by GSH-affinity chromatography followed by isoelectric focusing. The major enzyme (GST-7.64; 55% of total activity bound to the column) has a pI value of 7.64, is composed of two subunits each with a molecular mass of 23 kDa, and has the N-terminal amino acid residue blocked. GST-7.64 has also been characterized with respect to amino acid composition, substrate specificity, inhibition characteristics, c.d. spectra and immunological reactivity. The N-terminal sequence of some peptides obtained after tryptic digestion has also been determined. All together the results obtained suggest that the major toad liver GST is distinct from any known GST, including microbial, plant and mammalian GSTs.