1. Mercapturic acid pathway metabolites of propachlor, labelled with 14C, were incubated with pig caecal contents, small and large intestinal contents, and pure cultures of gastrointestinal bacteria. 2. The glutathione, cysteine, N-acetylcysteine, and the S-oxide of the N-acetylcysteine conjugates of propachlor (2-chloro-N-isopropylacetanilide) were each metabolized by mixed pig caecal micro-organisms to 2-mercapto-N-isopropylacetanilide. 3. The extent of formation of 2-mercapto-N-isopropylacetanilide by mixed pig caecal micro-organisms from mercapturic acid-pathway metabolites of 14C-propachlor was as follows: glutathione (43.4%), cysteine (32.6%), N-acetylcysteine (5.2%), and the S-oxide of the N-acetylcysteine conjugate of propachlor (6.1%). The S-oxide of the N-acetylcysteine conjugate of propachlor was converted by the mixed pig caecal micro-organisms to N-acetylcysteine and cysteine conjugates of propachlor. 4. Small intestinal contents metabolized the glutathione conjugate of propachlor to the cysteine conjugate; little C-S lyase activity was present in the small intestinal contents. 5. The cysteine conjugate of propachlor was metabolized to 2-mercapto-N-isopropylacetanilide by Fusobacterium necrophorum, Bacteroides vulgatus, Megasphaera elsdenii and Eubacterium aerofaciens.