The object of this study was to determine possible differences in the course of salmonellosis in mice maintained at 25° C and others kept at 5° C, and to uncover, if possible, mechanisms responsible for such differences. The LD50 dose for mice of Salmonella typhimurium, strain RIA, is 4.6 X 105 for animals individually housed without bedding and maintained at 25° C. It is 3.8 X 10³ for animals similarly housed but kept at 5° C. An intravenous injection of 0.1 ml of saccharated iron oxide (Proferrin) two hours prior to infection lowers the LD50 to 4.9 X 10³ and 4.0 X 10 for mice kept respectively at 25° C and at 50 C. Low environmental temperature and "blockage" of the reticuloendothelial system (RES) lower the resistance of mice to about the same degree, but low temperature and RES impairment together lower resistance as if each is acting independently. Doubling the volume of Proferrin more than doubles the change in susceptibility to infection manifested by the mice but this amount seems to be toxic for mice. Even more important is the incidence of staphylococci found in liver or kidney of mice infected with S. typhimurium and kept at 5° C. Cultures were made on animals that survived infection for a period of 14 days and, except for the largest challenge doses where only a few animals remained, the incidence of staphylococci was proportional to the number of salmonellae injected. At 25° C only a small percentage of mice have staphylococci in tissues and these occur independent of the infectious dose of salmonellae.