Studies on two strains ofL.monocytogenesisolated from human patients, have been reported. The 1949 and the 1951 strains are serologically similar but have slight biochemical variations. Cultures grown at temperatures ranging from 2.8 °C. to 5 °C. showed greater virulence and antigenicity as well as morphological differences when compared with cultures grown at 37 °C. Both the 1949 and 1951 strains were sensitive to penicillin in concentrations between 0.09 to 0.18 and 1.50 to 3.12 units per ml. respectively. They were not sensitive to sulphadiazine or sulphathiazole in concentrations up to 20 mgm. Per ml. Both strains produced an antigenic (lipoid-like) surface substance when extracted with chloroform and ether, which, when injected into rabbits, produced a monocytic response in the peripheral blood. A heat-killed vaccine protected guinea pigs from lethal doses of theListeriaorganism, whereas control guinea pigs injected with the same number of living organisms died with typical symptoms. Partial immunity is demonstrable by the fact that rabbits recovering following penicillin therapy resisted reinfection for a period of at least one hundred and ninety days. Further work is being carried on to determine whether there is a relationship between the heat-labile "L" surface antigen and virulence.