Since the original isolation of the type species of Leptospira ballum from a mouse, Mus musculus spicilegus,1 the natural occurrence of this serotype in colonies of Swiss albino mice has been demonstrated in Europe and the United States.2,3 In addition, L. ballum has been recovered from house mice, Mus musculus, and an opossum, Didelphis virginiana, in Virginia3; from man and Norway rats, Rattus norwegicus, in Puerto Rico 4; from Norway rats in British Columbia,5 and from rice-field workers in France.6 Although the first diagnosed human infection was contracted from laboratory mice,7 the handling of such infected mice had not been considered a serious health hazard for laboratory employees. The presence of L. ballum in mice in the breeding colony at the Rocky Mountain Laboratory was first demonstrated in 1949, but, until recently, human cases had not been recognized. In November, 1956, one member of the staff experienced a severe febrile