FREQUENCY OF DELETIONS AMONG SPONTANEOUS AND INDUCED MUTATIONS IN SALMONELLA

zoon coccoides has not been studied. The possibility that our tissue cultures from resistant mice are resistant because of latent infection in the mice themselves seems quite remote. The factors should not, in this case, segregate in a Mendelian fashion, and latent infection of young or newborn mice has not been reported in these infections. Summary.-A virulent strain of mouse hepatitis virus is shown to have a selective destructive effect on the macrophages cultured from the liver and other tissues of newborn mice, and no apparent effect on the fibroblasts and epithelial cells. Tissue susceptibility seems therefore to be a property of the reticulo-endothelial system. Cultures obtained from resistant strains of mice showed no destruction of macrophages, whereas susceptible strains of mice yielded macrophages which were destroyed in culture. Tests of hybrids resulting from crosses between resistant and susceptible strains indicate that susceptibility is inherited and that genetic segregation of susceptibility and resistance occurs in the F2 and backcross generations. This is apparent both in the mice themselves and in cultures obtained from the different genetic crosses.