As a consequence of previous experimental work concerning the sulphydryl (-SH) levels of target and non target tissues of various animal species subjected to a wide variety of chemical carcinogens Calcutt, Doxey and Coates (1960, 1961) have suggested that a rise in target tissue-SH levels during the tumour induction period is a prerequisite of tumour formation. Support for this viewpoint is given by Crabtree's (1944, 1945, 1946) findings that agents expected to interfere with cellular-SH are effective anticarcinogens. The present work was designed to examine the effects of some further polycyclic hydrocarbons on sulphydryl levels in mouse skin and also to see whether croton oil in its cocarcinogenic role influenced the-SH content of the tissue to which it was applied. The technique used for the measurement of skin-SH was identical with that used in earlier work. Full details are given by Calcutt and Doxey (1959) and Calcutt et al. (1960). EXPERIMENTAL Seventy Strong A male mice, aR fifteen weeks old, were divided into five groups of fourteen each. One group were untreated and served as controls for the remaining four groups. These were treated as below : Group I. Painted twice weekly with 0-2 ml. of 0-I per cent anthracene. Group II. Painted twice weekly with 0-2 ml. of 0-I per cent pyrene. Group 111. Painted twice weekly with 0-2 ml. of 0-1 per cent 1,2: 5,6dibenzanthracene.

THE role of sulphydryl (-SH) groups in both tumour growth and the induction of tumours has been a subject of interest for many years. Actual experimental work in this field has been limited and often only via indirect approaches. Methods of measuring the -SH within tissues have been unsatisfactory and in many cases involved tedious procedures with specialised equipment. Recently, Calcutt and Doxey (1959) described a simple technique for tissue -SH measurements using small weights of tissue. This technique was then used to determine the effects of various carcinogenic and non-carcinogenic hydrocarbons on the liver -SH values of mice previously treated with one of these hydrocarbons (Calcutt, Doxey and Coates, 1959). This work dealt with the relationships of-SH levels to the metabolism of the agents in question, since all are known to be metabolised in the liver but none can be regarded as a true hepato-carcinogen.The present paper records a study of the effects of known chemical carcinogens on the -SH levels of susceptible and non-susceptible tissues. We have been influenced in this choice of subject by knowledge of two previous papers which bear upon this field. Boyland and Mawson (1938) found that intraperitoneal injection of 3: 4: 5: 6 dibenzocarbazole (a known hepato-carcinogen) causes a considerable rise in liver glutathione which is persistent over several months. The livers of mice injected with methylcholanthrene or 1 : 2: 5: 6 dibenzanthracene (neither being a liver carcinogen) did not show a similar increase in glutathione. Di Paolo and Niedbala (1957) found that after a single painting with either 1: 2: 5: 6 dibenzanthracene or 9:10 dimethylbenzanthracene the -SH content of mouse skin was raised above the normal value and this increase was persistent for at least five days. The chemically related, but non-carcinogenic compound, anthracene, was found to have no effect on skin -SH levels.

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AN examination of the effects of several chemically unrelated carcinogens upon the sulphydryl (-SH) levels of various susceptible and non-susceptible tissues led to the question of an increase in -SH levels being an essential feature of tumour induction (Calcutt, Doxey and Coates, 1960 At intervals over a period of ten weeks the control animals were killed and the -SH levels of liver, kidney and lung were measured. Corresponding measurements were also made in respect of the experimental animals, these being killed singly daily for four days and then at weekly intervals over the period of ten weeks.The results for the liver are shown in Fig. 1. Since no pattern was discernible in the figures for the control animals these have been plotted as a mean and standard deviation. It will be seen that the -SH level of the experimental livers rose above the control figure 48 hours after commencement of the experiment and then remained persistently high. In this and all subsequent experiments -SH levels are expressed as ,sg. of -SH per 100 mg. wet weight of tissue.

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