Conjugation and cleavage products in the thiolytic metabolism of the anticancer drug 4'-(9-acridinylamino)methanesulfon-m-anisidide were identified primarily by high-pressure liquid chromatography in combination with field desorption mass spectrometry. The spontaneous metabolic pathway of the drug, as related to its susceptibility to nucleophilic attack by endogenous thiols at the 9-carbon atom of the acridine moiety, has been studied. Among the metabolite fraction of 4'-(9-acridinylamino)methanesulfon-m-anisidide excreted in rat bile after administration of a therapeutic dose, a conjugate was identified as the 9-acridinyl thioether of glutathione. This conjugation product and the corresponding 9-acridinyl conjugates were formed spontaneously after incubation of 4'-(9-acridinylamino)methanesulfon-m-anisidide with glutathione, cysteine and N-acetylcysteine in sodium phosphate buffer and other aqueous media, as established by high-pressure liquid chromatography and field desorption mass spectra. The thiolytic pathway results in the release 4-amino-3-methoxymethanesulfonanilide which was identified in all in vitro experiments and in rat serum after intravenous 4'-(9-acridinylamino)methanesulfon-m-anisidide. 9(10H)-Acridone, 9-aminoacridine and other acridine derivatives which occur as minor products during the thiolytic cleavage in vitro were identified by field desorption and partially by high resolution electron impact mass spectrometry.