Mouse liver glutathione transferase P1-1 has three cysteine residues at posi-tions 14, 47 and 169. We have constructed the single, double and triple cys-teine to alanine mutants to define the behaviour of all three thiols. We confirm that C47 is the ‘fast’ thiol (), and define C169 as the alkaline reactive residue with a pKa of .