The LD₅₀ of the test microorganism in each challenge was estimated from the mortality data.

Exposure to 300 r caused little increase in susceptibility to this experimental infection. 400 r caused a moderate increase on the 3rd day post-irradiation with return to normal on the 17th. Leucocyte counts (geometric means) following this dose of radiation did not fall below 1200.

500 r caused a marked increase in susceptibility which lasted from the 3rd to 11th day, during which period the leucocyte counts were below 800. On the 20th day, susceptibility to infection was normal, although the geometric mean of leucocyte counts was only 2200.

Comparison of mortalities resulting from equivalent inocula introduced by the two routes (intravenous and intraperitoneal) showed no difference in unirradiated mice. However, among mice irradiated with 400 or 500 r, higher mortalities resulted from intravenous inoculation. The difference was found to be due to the establishment of a small focus of infection at the site of intravenous injection as a result of leakage of a minute fraction of inoculum into the perivenous tissues of the tail. Bacterial multiplication occurred in such foci in irradiated (leucopenic) mice, but not in unirradiated mice, nor at the site of intraperitoneal inoculation even in irradiated mice.