302. Cosgrove et al. [C23] tested on (101xC3H)F1 female mice following a wide range of x-ray exposures (300 - 1800 R) the effect of the radioprotective drugs aminoethylthiouronium (AFT) with or without parallel treatment with isologous bone marrow infusion. The drug treatment was found to have a marked protective effect against early lethality, but its effectiveness in protecting against a reduction in longevity was equivocal. No effect was found on tumour induction, nephrosclerosis and lens opacities while induction of thymic lymphomas and graying of the fur were reduced by the drug treatment. Thus AET protected against some but not all long-term somatic effects and in no case the dose reduction factor approached that obtained against the acute lethal effects of radiation (40-50 per cent). 303. In another experimental series performed on male and female LAF1 mice by the same workers [C20] administration of AET before irradiation led to some re- duction of kidney sclerosis but was again without effect in regard to the induction of tumours of the ovary or to the graying of the fur. 304, An attempt to maximizing protection against 9 MeV irradiation was repor- ted by Shewell and Wright [S33] who combined four different methods of protection: administration of cysteamine before irradiation, irradiation during ni- trogen hypoxia, and administration of syngeic bone marrow and antibiotics after irradiation. The LD for the protected mice (C3H/Bi, 15 weeks old) was in- creased by a factor ae with respect to unprotected animals and this factor persisted throughout the long-term follow-up of the mice surviving early lethality. Graying of the hair and epilation also gave a dose-reduction factor of 3.8, but the appearance of radiation cataracts did not conform to the same pattern. It was therefore concluded that the protection afforded against the different effects had variable dose-reduction factors for each effect tested. 305. In Nelson's [N10] experiments irradiation followed various fractionation schedules: 80 R at intervals of 1 day up to total accumulated exposures of 646 1920 R; 80 R at intervals of 3 days for the whole life-span; 160 R at intervals of 1, 3 and 7 days up to exposures of 480 - 1760, 1600 - 3250 and 2880 - 5760 R, respectively. Cysteamine at 4 mg/day for 24 days or at 4 mg/day twice a week was used as a chemical protector. The drug treatments by themselves, as well as the injection of physiological saline twice a week for the whole life, modified drastically the mean and median survival time of the irradiated animals. However,