Leptospira isolated from patients and natural paddy water were further studied to confirm their serologic specificity with the bacteria in infected animal tissue and autopsied tissue of patients died with leptospirosis. And pathologic patterns of the inoculated antimal and the virulence of the bacteria in the animals were also studied. The findings are summarized as follows; $LD_{50}$ dose of the bacteria in guinea-pig were $1-2{ imes}10^9$ cells and mice were found to be susceptible to them even though the degree of susceptibilities were much inferior to guinea pig. The bacteria were recovered from various organs; demonstrated massive hemorrages due to diapedesis and monocyte infiltration were observed, in some cases, intramedullary hemorrages of the infected kidnies and hematuria were recognized. All of the hyperimmune sera were strongly reacted with the tissues from autopsied human case who died of EPHF(Epidemic Pulmonary Hemorrhagic Fever) in immunofluorescent antibody tests.