The microsomal glutathione S-transferases (MGSTs) catalyse the nucleophilic attack by reduced glutathione (GSH) on nonpolar compounds that contain an electrophilic C, N, or S atom. Three major families of proteins are widely distributed in nature. The cytosolic and mitochondrial GST families comprise soluble enzymes that are only distantly related whilst the third family comprises microsomal GST, referred to as membrane-associated proteins in eicosanoid and glutathione (MAPEG) metabolism. Three members of this family function as detoxification enzymes, MGST1-3 (DeJong et al. 1988, Kelner et al. 1996, Jakobsson et al. 1996, Jakobsson et al. 1997). Electron crystallography studies in rat Mgst1 indicate these enzymes function as homotrimers (Holm et al. 2002). Both aflatoxin B1 exo- and endo-epoxides (AFXBO and AFNBO) conjugate with glutathione. These conjugates are eventually excreted in urine as mercapturic acids.