Tests on hamsters reveal that there is a decided protective index from the soluble antigen prepared by the acid-heat extraction of living cultures of Leptospira pomona. They also reveal that the antigen must be given in quantities sufficiently small to be amenable to the system of the test animal or a condition of ”immunogenic paralysis“ results. Due to the number of hamsters in any one group whose pooled sera was tested, a fair evaluation could not be made between various groups in the agglutination-lysis test. Tests on the stability of the immunogenic agent used show that it is completely stable for at least a period of three months when stored in a refrigerator at 5 °C. / Master of Science