Evidence is presented that ligandin, an intracellular protein involved in the binding of such anions as bilirubin, indocyanine green, and penicillin, is identical to glutathione S-transferase B (EC 2.5.1.18), an enzyme catalyzing the conjugation of glutathione with such electrophiles as 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, iodomethane, ethacrynic acid, and bromosulfophthalein. The proteins, isolated by distinct methods, have the same specificity for substrates and for ligands, react in identical fashion to antibody produced against ligandin, bear entirely similar physical characteristics and amino acid composition, and are both induced in response to phenobarbital. Indocyanine green, one of the ligands that is not effective as a substrate, was shown to competitively inhibit the conjugation reaction. It is suggested that specificity is directed toward compounds with electrophilic sites.