1. The role of metabolites in sulphasalazine-mediated toxicity has been investigated in vitro by the use of human red blood cells and mononuclear leucocytes as target cells, with methaemoglobin formation and cytotoxicity respectively, being the defined toxic end-points. 2. Of the metabolites of sulphasalazine investigated, only sulphapyridine was bioactivated by human liver microsomes in the presence of NADPH to a metabolite which caused marked methaemoglobinaemia and a small, but statistically significant degree of mononuclear leucocyte cell death. 3. Methaemoglobinaemia was inhibited by ketoconazole but not by ascorbic acid (100 microM), glutathione (500 microM) and N-acetylcysteine (50 microM). In contrast, ascorbic acid and the thiols afforded complete protection for mononuclear leucocytes. 4. Sulphapyridine (100 microM) was converted in vitro to a metabolite (metabolite conversion 6.8 +/- 0.3%), the retention time of which on h.p.l.c. corresponded to synthetic sulphapyridine hydroxylamine. The half-life of sulphapyridine hydroxylamine in phosphate buffer (pH 7.4) was found to be 8.1 min. 5. In the absence of microsomes and NADPH, sulphapyridine hydroxylamine caused a concentration-dependent (10-500 microM) increase in methaemoglobinaemia (2.9%-24.4%) and cytotoxicity (5.4%-51.4%), whereas sulphasalazine, sulphapyridine, 5-hydroxy sulphapyridine and 5-aminosalicylic acid had no effect.