The author studied the general natures, immune reactions, quantitative cultivations and pathogenicity of the original avian A strain cultured on the glycerine media and of the variant strain cultured on the plain broth agar media. The cells cultured for 10 days were used, and fowls and rabbits were used as the test animals. The results were as follows: 1) When a small quantity of the cells was inoculated into fowls, no remarkable difference was observed between the original and variant strains except the pathological appearances of the liver and spleen. 2) When a large quantity was inoculated, the positive reaction rate and grade of tuberculine test were high in the original strain-inoculated group, and, in quantitative cultivation test in lung, far more multiplying cells were observed in the original strain-inoculated group than in the variant strain-inoculated group. 3) From the inoculated rabbit blood, the variant strain disappears earlier than the original strain. No remarkable difference was, however, observed in the pathological appearances of the inoculated rabbits. 4) In both groups inoculated with each of the original and variant strains, the chief site of the pathological change was the liver. In case of a large quantity-inoculation, the chief site of the pathological change and the result of the quantitative cultivation did not always accord with each other. 5) The used original standard strain is of very weak virulence, and, accordingly, the determination of the difference of virulence between the original and variant strains is possible only by the general observation of all results of the animal experiments.