The author, by sensitizing penicillin on the certain rabbits, that have been decreased in antibody production by splenectomy, indiaink blockade and carbon tetrachloride injection, has proved by experiment Congored Index, electrophoretic fraction of serum protein, blood picture, penicillin-sensitized erythrocyte agglutination reaction and local hypersensitivity. As a result, the author has found the decrease of sensitivity for penicillin of the rabbits.