Mice immunized against cancer by means of an injection of defibrinated blood show in the germinal centers of the lymphoid organs a marked increase in the numbers of mitotic figures. The increase becomes evident 48 hours after the injection in the majority of instances and reaches its climax at about the 5th day. After this time it subsides, returning to the normal rate about the 10th day. These immunized animals, when inoculated with a cancer graft 10 days after the injection, show a second stimulation of the lymphoid centers similar to the first but more intense in character. This increase in the number of mitotic figures becomes evident as early as 24 hours after the cancer inoculation and persists in a marked degree for a week, after which there is a gradual return to the normal rate. The lymphocytes of the circulating blood during the establishment of the immunity show frequent examples of amitotic division, and many examples of irregular and lobulated nuclei. These changes suggest intensified functional activity. Contrary to the statement of Da Fano, cellular reaction in the subcutaneous tissues of immunized animals is present only in the region infiltrated by the injected cells. This fact becomes conspicuous when the immunizing injection is given intraperitoneally, in which case no cellular accumulations are noted in the loose connective tissues. No constant cellular changes were noted in the bone marrow, thymus or thyroid gland, liver, or kidney of the treated animals.