Aflatoxin B1 (AFB1)-8,9-oxide, the proposed ultimate carcinogen is conjugated enzymically with glutathione (GSH) to give 8-(S-glutathionyl)-9-hydroxy-8,9-dihydro AFB1 (AFB1-SG). The GSH conjugate isolated from rat bile was shown, on the basis of 1H n.m.r. to be identical to AFB1-SG. Of the seven soluble rat liver GSH transferases tested, namely GSH transferases 1-1, 1-2, 2-2, 3-3, 3-4, 4-4 and 5-5 (see reference 1 for the new system of nomenclature), only the first three were active with microsomally generated AFB1-8,9-oxide, their rates of conjugation being 1.1, 0.61 and 0.64 nmol/min/mg enzyme, respectively. AFB1-SG is a thioacetal, but it was not formed from the incubation of the hemiacetal, AFB1-8,9-dihydrodiol, with GSH or GSH plus GSH transferase 1-1 plus 1-2. The covalent binding of in vitro microsomally activated AFB1 to DNA and the formation of AFB1-SG were linearly related to AFB1 concentration in the range of 0.2-2 micrograms/ml. DNA binding was decreased by 38% by the competing formation of AFB1-SG throughout this range of concentrations. These results are in accord with the observation of Scott Appleton et al. (Cancer Res., 42, 3659-3662) that, in the rat in vivo, there is no evident threshold for the binding of AFB1 to DNA. These findings are also consistent with the further observation, reported in this paper that GSH and GSH transferases have no effect on the mutagenicity of microsomally activated AFB1 to Salmonella typhimurium TA 100.