1. The cleavage of sulphasalazine at the azo bond by bacterial suspensions and tissue homogenates has been studied in vitro.
2. For maximum activity the azo reductase system requires anaerobic conditions and the presence of cofactors, namely NADPH and FAD. in this respect, sulphasalazine resembles other azo dyes.
3. Under optimum conditions all the species of bacteria tested were capable of splitting sulphasalazine and there were no major differences in the degree of activity shown by different species. The enzyme system is located within the bacterial cell and does not leak out of it.
4. All the tissues tested, both human and rat, showed azo reductase activity. The liver showed a much higher activity than the other tissues.