Materials and Methods

Animals.--Cox Swiss mice (males, 4-5-wk old), obtained from Ckafles River Laboratories, Wilmington, Mass., were used in these experiments. Organisms.--The strain of Listeria monocytogenes used was obtained from Dr. GB Mackaness. Suspensions of the bacteria were prepared from an 18 h culture of Listeria grown in Trypticase soy broth (Difco Laboratories, Inc., Detroit, Mich.). The organisms were washed twice and resuspended in sterile saline to give the approximate number of bacteria per milliliter as determined by the optical density. The suspensions (or dilutions of the suspension) were plated and bacterial colonies counted to determine the specific number of Listerla in the original suspension. The LDs0 for this strain when iniected intravenously was found to be 5>(10 microorganisms. A sublethal dose of Listeria, 1.5 X 104, was injected intravenously into the mice.