SUMMARY.-The acid soluble disulphides and mixed disulphides of a range of normal rat and mouse tissues and a number of transplanted rat or mouse tumours were measured. The results were considered in relation to other workers' data. It is noted that the more radioresponsive tissues have higher levels than the more radioresistant tissues.FOR a long time it was generally considered that the level of acid soluble disulphides in tumours was very low. Tarnowski, Barclay, Mountain, Nakamura, Satterwhite and Solney (1965) used a newly described reduction system-by means of sodium borohydride-and concluded that the concentration of acid soluble disulphides was comparable with the concentration of acid soluble thiols. This conclusion, however, was not warranted since Tarnowski et al. carried out their reductions on whole homogenates of tumours and then after deproteinisation estimated the thiol level for a comparison with that derived from a similar system without the reducing step. They therefore measured acid soluble disulphides plus any small acid soluble thiol groups broken off from proteins during the reduction process; that is the usually described " mixed disulphides ".Based on data derived from experiments with the Ehrlich ascites tumour Revesz (1969) has concluded that the acid soluble disulphides exist at a low level as compared with the mixed disulphides.To clarify this situation we have undertaken a new series of measurements of both acid soluble and mixed disulphides in a number of transplanted mouse and rat tumours. For comparison similar measurements have been made on a variety of normal mouse and rat tissues. METHODS Acid soluble disulphidesHaving considerable experience of the measurement of sulphydryl (-SH) groups by the technique described by Calcutt and Doxey (1959) andCalcutt, Doxey andCoates (1960) the system of measuring the difference between acid soluble thiols with and without reduction was preferred. Test runs involving a variety of reducing systems gave unsatisfactory results. The only method found to be repeatable and reliable was to homogenise a weighed amount of tissue in the presence of EDTA and o dipyridyl as described by Calcutt and Doxey (1962) and then to precipitate proteins with trichloracetic acid. After removal of the protein the acid solution

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