BULLETIN OF THE TORREY BOTANICAL CLUB 312 [Vou 91 had essentially the same average Sr-90 content (ca. 100 me/mi?) as samples collected in 1957 immediately after contamination. Of the 43 animals collected one year after fallout (1958), the bone ash of 41 contained Sr-90 equivalent to > 10 Sr Units. Of the 53 animals collected 3.7 years after fallout (1961), the bone ash of 39 contained Sr-90 equivalent to < 10 Sr Units, and only 14 contained Sr-90 equivalent to > 10 Sr Units. All the 1961 animals found to have higher levels of Sr-90 in their bones were judged, on the basis of weight, to be members of the older age groups. Since the average life-span of jack rabbits is about 3.2 years, the older animals collected in 1961 could have been present during the first year after fallout. Most of the animals collected in 1958 probably were present at the time of fallout. Table 10. Sr-90 in jack rabbits and soils of areas contaminated by fallout from the Nevada Test Site*., . Miles Sr Units in Sr-90 me/mi? in Soils from NTS JackRabbit Total** <1 2 80 140 205 350 50.4 23.9 215 13.8 12.9 20.9 9014 933 142 41 32 67 one ota Acid Percent 980 58 18 27 16 48 10.9 6.2 13.0 66.7 50.3 7138 Soluble*** Soluble * Samples collected in 1958. ** Based on sodium carbonate fusion method of analysis *** Based on extraction with 6 N HCl After: Larson et al. (1962). Table 10 illustrates the relationship between Sr-90 in the soils of areas contaminated by fallout from the Nevada Test Site and the concentration of Sr-90 in the bones of jack rabbits living in those areas. As indicated by these data, the highest levels of Sr-90 in rabbit populations are associated with higher concentrations of Sr-90 in soils, but the relationship is not linear. Large increases in the total or acid soluble Sr-90 content of soils may be accompanied byrelatively small increases in the Sr-90 content of jack rabbit boneash. 4, Discussion. While the results of these and similar studies in other areas close to the Nevada Test Site are not conclusive, they may be explained mn relation to the following observations. Both solubility and particle size have been suggested (Bryant et al. 1960, Lindberg et al. 1959) as indices of the ‘‘biological availability’’ of Sr-90 fallout. The ‘‘availability’’ of Sr-90 to the animals living in a fallout contaminated area depends on ‘‘availability’’ for ingestion and on ‘‘availability for assimilation’’ during its passage throughthe eut. It has been demonstrated that Sr-90 is associated primarily with fallout particles < 44p