1. Mast-cell tumour P815 cells oxidize [35S]cyst(e)ine to 35SO42−. 2. Addition of cysteinesulphinate or sulphite decreases the formation of 35SO42−; at the same time [35S]cysteinesulphinate or 35SO32− accumulates. 3. Extracts of P815 cells form sulphite from cysteinesulphinate and 2-oxoglutarate. The Km for cysteinesulphinate is 6·35mm and that for 2-oxoglutarate is 0·165mm. 4. Extracts oxidize sulphite to sulphate. 5. No formation of hydrogen sulphide from cyst(e)ine was detectable. 6. It is concluded that P815 cells oxidize cyst(e)ine to sulphate solely via cysteinesulphinate and sulphite. 7. The concentration of the enzymes catalysing this sequence is unaltered by several variations in the conditions of growth.

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