The conjugation of 1-p-chlorophenyl-4,4-dimethyl-5-diethylamino-1-penten-3- one hydrobromide (CDDP), a Mannich base of an alpha,beta-unsaturated ketone, to glutathione is catalyzed selectively by alpha-class glutathione S-transferase. The reaction of CDDP with glutathione can be monitored continuously by following the conjugation-dependent decrease in absorbance of CDDP at 307 nm. The Km and Vmax for CDDP with alpha-class glutathione S-transferase from horse liver were determined to be 226 microM and 14.6 mumol/(min.mg), respectively. CDDP is the first example of an alpha-class glutathione S-transferase selective substrate that monitors the glutathione conjugation activity, rather than the glutathione peroxidase activity of the enzyme.