gle parameter. A mathematical model based on the induction of cytogenetic da- mage was compatible with the findings and suggested that species susceptibility to continuous exposure could be related to an accumulation time for damage leading to chromosome breaks. At this level of analysis the actuarial para- meters could be linked with the study of molecular lesions as discussed under paragraphs 54-55, although filling the gap between the two approaches requires clearly much confirmatory work. 2. 238. Intra-species differences The oldest contribution to the problem of strain differences in the re- sponse of mice to long-term radiation effects is that of Gowen [G8]. He studied two strains, the S mouse with a medium longevity and the Ba mouse with a long life-span. Upon irradiation in the course of a nuclear test (30 - 360 rep ex- posures) the S mouse were shown to be considerably more resistent than the Ba ones and this pattern of response was similar to that observed after 98 kVp x-irradiation (exposures of 20 - 960 R). There were no good pathological ob- servations reported, and it was argued that the resistance of the S strain to Salmonella might at least partly account for the differences observed, although the full basis of the radioresistance of these mice was thought to be more complex. 239. Grahn [G10] reported on the chronic lethality of five mouse strains: Balb/c, A/Jax, A/He, C3H1/He and C57BL/6. ray acute exposures The survivors of single 200 kVp x- (400 to about 700 R) were kept for analysis of late effects. Weighted regression lines of life-shortening on dose showed no significant differences between the strains in either sex, although females were more sen- sitive per unit dose. There was no correlation between life-span reduction and control life expectancy, although such a relationship could be shown in respect to LD In a second series daily 605, gamma exposures were tested at eleven exposurelevels between 220 R/day and 6 R/day, covering the full range from acute to chronic responses. were also included. Partial data from crosses of Balb/c x C57BL/6 Preliminary data showed strain differences at all expo- sure levels; survival times were quite variable between strains but closely reflected genetic differences in control survival. The greatest life-shortening effect was seen in strains with a high incidence of leukaemia. ue