Rat liver mitochondrial aspartate aminotransferase (a homodimer) was shown to catalyse a β-lyase reaction with three nephrotoxic halogenated cysteine S-conjugates [ S -(1,1,2,2-tetrafluoroethyl)- l -cysteine, S -(1,2-dichlorovinyl)- l -cysteine and S -(2-chloro-1,1,2-trifluoroethyl)- l -cysteine], and less effectively so with a non-toxic cysteine S-conjugate [benzothiazolyl- l -cysteine]. Transamination competes with the β-lyase reaction, but is not favourable. The ratio of β elimination to transamination in the presence of S -(1,1,2,2-tetrafluoroethyl)- l -cysteine and 2-oxoglutarate is >100. Syncatalytic inactivation by the halogenated cysteine S-conjugates is also observed. The enzyme turns over approx. 2700 molecules of halogenated cysteine S-conjugate on average for every monomer inactivated. Kidney mitochondria are known to be especially sensitive to toxic halogenated cysteine S-conjugates. Evidence is presented that 15—20% of the cysteine S-conjugate β-lyase activity towards S -(1,1,2,2-tetrafluoroethyl)- l -cysteine in crude kidney mitochondrial homogenates is due to mitochondrial aspartate aminotransferase. The possible involvement of mitochondrial aspartate aminotransferase in the toxicity of halogenated cysteine S-conjugates is also discussed.