The C3H mouse liver h-protein is a cytoplasmic protein to which metabolites of carcinogenic polycyclic hydrocarbons bind covalently following i.p. injection. It has a number of physical properties similar to those of the glutathione S-transferases (EC 2.5.1.18). These properties include MW (40,000), number of subunits (2), basic isoelectric point around 8.0, sedimentation coefficient (3.5 S) and subcellular localization. Glutathione S-transferase activities with 1,2-epoxy(3-p-nitrophenoxy)propane and benz[a]anthracene-5,6-oxide as substrates were separated from the h-protein on carboxymethylcellulose and isoelectrofocusing columns. Purification of the mouse h-protein as a [3H]-7,12-dimethylbenz[a]anthracene conjugate or as the free form is also described.