243, The problem of the genotype relationship to long-term survival has also been considered by Holland and Mitchell [H10] who irradiated (300 R, 300 kVp x rays) four strains of male and female inbred mice (C3Hf/Wg, C57BL/6, RFM/Un, Balb/c), C3CF1 and B6FRMF1 hybrids and a cross between these two hybrids, C3CB6RFM. Females were shown to be, in general, more sensitive than males by a factor of 1.5. Within the same sex, substantial differences in sensitivity were also shown among strains. Since these differences correlated well with the weight of the animals, it was suggested that the variation in susceptibi- lity to life-shortening might be at least partially accounted for by genetically-determined differences in the maturation rates of the various genotypes. In fact, during growth the weight of the animals may be an expression of the maturation rate rather than of the general state of health of the animals, as it is during adulthood. 24hy. After fission neutron irradiation at both high (25 rad/min) or low (1 rad/day) dose-rate no appreciable difference in life-shortening was found be- tween RFM or Balb/c female mice in the experiments of Ullrich and Storer [U8]. In another report by Spalding et al. [S22] experiments in progress were re- ported on RF/J and C57BL/6J mice, whose results will be added to the present document as soon as they will become available. B. aks. SEX AND BODY WEIGHT Very often in the course of the papers reviewed mention is made of dif- ferential effects in the two sexes for various strains of animals. To review separately all these observations would probably be unnecessary since in many cases they are incidental and not particularly relevant to the main physical or biological variable discussed in each specific paper. In the following paragraphs (246-254) only those papers are reviewed where such effects were particularly well substantiated and discussed. Emphasis will be given to con- tributions where pathological observations were performed, in an effort to ascribe the differential effects to diseases or conditions affecting preferen- tially one of the sexes, 1. 2ueé. Sex In the experiments of Neary, Munson and Mole [N3] (see paragraphs 111-112) the mean survival time of the female animals was significantly greater at the