SummaryInvestigations have been carried out on the effects of the radiosensitizing and antimitotic compound, 2-methyl-1,4-naphthaquinol bis (disodium phosphate) [MNDP] and its tritiated analogue on the sulphydryl (−SH) levels of two strains of mouse ascites tumour in vivo.Treatment with MNDP gave a decrease in acid soluble −SH after 30 min in vivo. In this time-interval tritiated MNDP produced a much greater decrease than MNDP alone or a combination of this compound and tritiated water. No change in disulphide levels was apparent.A similar drop in the total −SH levels was found, and in some cases this was directly proportional to the dose of MNDP given. 2,3-Dimethyl-1, 4-naphthaquinol bis (disodium phosphate) and vitamin K1 bis (disodium phosphate) gave slight increases in −SH levels, indicating that position three of the quinone ring must be free for combination with −SH groups. No change in total −SH was apparent 30 min to 1 hour after treatment of this tumour with tritiated MNDP, but a definite decreas...