Microbe G, rabbit septicemia bacillus, hitherto found only as a mutant in cultures of the rabbit septicemia bacillus, Type D, has been demonstrated to exist on the nasal mucous membranes of normal rabbits. This organism corresponds in lack of virulence, character of growth, acid agglutination optimum, and immune agglutination reaction, to the mutant G form described in previous papers. Microbe D has been found to be present on the nasal mucous membranes of normal rabbits. These animals have survived for months with no evidence of infection other than the presence of the organism. These Type D organisms, despite their failure to cause fatal damage in their own host, are shown to possess the typical high virulence characteristic of this type, when injected intrapleurally into young rabbits. Rabbits which are carriers of Type D or G possess a definite amount of immune agglutinins, as evidenced by test of their serum against Microbe G at pH 7.1. Rabbits free from infection with these organisms invariably have yielded a serum which fails to agglutinate Type G completely in 1:10 or higher dilutions.