Aflatoxin B1 (AFB1) requires bioactivation to AFB1-8,9-epoxide for carcinogenicity, and glutathione S-transferase (GST)-catalyzed conjugation of activated AFB1 with glutathione (GSH) is a critical determinant of susceptibility to the mycotoxin. Incubations containing [3H]AFB1, rabbit liver microsomes, an NADPH-generating system, 1 mM GSH, and GST-containing lung or liver cytosol were performed to assess the abilities of lung and liver GSTs to conjugate AFB1-8,9-epoxide. [3H]AFB1-GSH was isolated by isocratic reverse-phase high-performance liquid chromatography (HPLC) and quantitated by liquid scintillation spectroscopy. Maximal [3H]AFB1-GSH formation rates were significantly lower for lung than for liver (0.3 +/- 0.1 and 1.7 +/- 0.4 nmol/mg/hr, respectively). Immunoprecipitation of rabbit pulmonary cytosolic GSTs with anti-alpha or anti-mu GST antisera decreased [3H]AFB1-GSH production by approximately 45 and 51%, respectively, indicating that alpha-class and mu-class GSTs are of similar importance in catalyzing this reaction in the lung. Because mu-class GSTs comprise only a small proportion of total lung GST content, these enzymes have high specific activity toward AFB1-8,9-epoxide. In contrast, the pi-class GST appeared to play a negligible role. Using a rat liver microsomal system to generate both AFB1 exo- and endoepoxide isomers, and analysis based on chiral HPLC, we found that rabbit liver cytosolic GSTs catalyzed formation of both AFB1 exo- and endo-epoxide-GSH conjugates, whereas pulmonary cytosolic GSTs catalyzed formation of only the exo stereoisomer at detectable levels. Despite a preference for conjugating the more mutagenic AFB1 exo-epoxide isomer, the relatively low capacity for GST-catalyzed detoxification of bioactivated AFB1 in lung may be an important factor in the susceptibility of the lung to AFB1 toxicity.