Summary: With the blood serum of tuberculous guinea pigs, hypersensitive to eggwhite, it was possible to transfer the hypersensitiveness passively. In the passively sensitized guinea pigs, both tuberculous and non-tuberculous, only the large evanescent type of reaction was observed. A sensitiveness toward tubercle bacillus extract, similar in type to the reactions of guinea pigs passively sensitized toward eggwhite, could be transferred with the sera of tuberculous guinea pigs treated with tubercle bacillus extracts. The passive sensitiveness increased in strength during several days following the serum injections.