Glutathione (GSH) S-transferase (GST) isoenzymes of the small intestine and colon of female A/J mice have been purified and characterized to determine their interrelationships with other murine GSTs. Cytosolic GST activity in the small intestine was at least due to six isoenzymes with isoelectric points (pI) of 9.5, 9.3, 9.1, 8.5, 6.2 and 5.5. Small intestine isoenzymes with pI values of 9.5, 9.3, 8.5, and 6.2 were identical to the mGSTA1-1 (Alpha class), mGSTP1-1 (Pi class), mGSTM1-1 (Mu class) and mGSTA4-4 (Alpha class), respectively, of other A/J mouse tissues on the basis of their reverse-phase HPLC elution profile, immunological cross-reactivity and/or N-terminal region amino acid sequence. Even though GST9.1 of the small intestine cross-reacted with the antibodies raised against Pi class GST, reverse-phase HPLC and N-terminal amino acid sequence analyses suggested that this isoenzyme may be structurally different from mGSTP1-1 as well as mGSTP2-2. Likewise, despite immunological similarity with the Mu class GSTs, small intestine GST5.5 appeared to be different from other Mu class murine GSTs characterized previously. Cytosolic GST activity in the colon was mainly due to four isoenzymes with pI values of 9.8, 9.4, 6.6 and 5.8. While the identity of colon GST6.6 could not be established due to its low abundance, GST9.8, GST9.4 and GST5.8 were identical to mGSTP1-1, mGSTM1-1 and mGSTA4-4, respectively, of other A/J mouse tissues including the small intestine. Isoenzymes corresponding to small intestine GST9.1 and GST5.5 could not be detected in the colon. The results of the present study indicate that the small intestine of female A/J mice is better equipped for protection against toxic effects of electrophiles than colon.