302 O. C. A. SCOTT 



long-term effects. It may be of importance that cysteine alone in the diet, 

 prolonged the life of mice (Harman, 1957). 



HoUcroft et al. (1957) investigated the effect of cysteine combined with 

 partial anoxia (6% oxygen) on tumour incidence in mice after 400 and 900 r. 

 The protective treatment appeared to give a DRF of 2-5, in so far as the pro- 

 tected group receiving 900 r, showed the same tumour mcidence as that in the 

 400 r sham spleen-protected mice. However, complete dose-response curves 

 are not available and a third group, receiving 900 r + spleen protection 

 showed a lower tumour incidence than the 400 r spleen-protected group. This 

 effect may be due to earlier deaths at the higher radiation dose. There is a 

 need for a correction factor to allow for the age effect, and Dr. Upton may be 

 able to explain Dr. Kimball's correction factor, as applied to their own data. 

 Mewissen and Brucer (1957) investigated the effect of cysteamine and 

 cystamine on the incidence of radiation-induced lymphosarcoma, and lym- 

 phatic leukaemia m C57 black mice. The incidence of tumours was higher, at 

 low y-ray doses, in the treated than in the control groups, but the meaning of 

 this result is not clear. 



Maisin et al. (1957) found that mercaptoethylamine had no effect in 

 irradiated rats, but the dose they used may have been too small (10 mg/145- 

 160 g rat). 



Upton et al. (1959) tested MEG in irradiated mice. There was a slight, but 

 as they themselves point out, not statistically significant, reduction in the 

 incidence of granulocytic leukaemia. The reduction in numbers of thymic 

 lymphomas was rather more convmcing. 



The genera] picture of chemical protection vis-a-vis carcinogenesis is thus 

 far from clear. On the other hand, chemical agents give definite protection 

 against other long-term effects; von SaUman et al. (1952) and Pirie and Lajtha 

 (1959) have shown that cataract formation can be inhibited by cysteine given 

 before irradiation. 



The many mteresting examples of chemical protection of the skin wiU no 

 doubt be discussed by Professor Bacq at this meeting. 



Partial anoxia has been shown to protect animals against some long-term 

 effects in mammals. Lamson et al. (1958) consider that the effect of 1,000 r 

 delivered to rats breathing oxygen is equivalent to 425 to 500 r delivered in 

 air. Krebs and Brauer (1961) also investigated the effect of 5% oxygen, 

 usmg mice and found no significant protection against the non-recouperable 

 element of radiation injury. 



Wright (personal communication) points out that the failure to protect a 

 particular system by means of 5% oxygen may be due to the fact that the 

 target-organ is not rendered anoxic by this procedure. Lindop and Eotblat 

 (1962) have subjected mice to 25 seconds complete anoxia. They foimd very 

 good protection against the acute effects (DRF of •-^ 2-8 in 4 to 6 week-old 



