The experiment was designed to clarify antibody-forming activity as an immunological function of the palatine tonsil and to compare its histologic findings with those of the other lymphoid organs by autoradiographic method. For this experiment, 47 young adult rabbits were used and randomly divided into 3 groups : Group I (experimental group) received a single injection of 22 Lf diphthelia toxoid and of thymidine-H³ intravenously. Group II (experimental conrol group) received only a single injection of thymidine-H³ intravenously. Group III (normal control group) were untreated. These animals were sacrificed by bleeding at various intervals to get sera for antibody titration and the palatine tonsils for histological studies. The paraffin sections were stained with hematoxylineosin, and methyl-greenpyronin. The slides were examined under the light microscope. The results were as follows : 1) In the experimental group, anti-diphthelia toxoid was detected within 24 hours in their sera after injection of the antigen. There was a rapid increase in antibody synthesis until 6 days and then gradual decrease until 3 weeks. 2) At 24 hours after injection of the antigen, the large pyroninophilic cells were observed in the extrafollicular area of the palatine tonsils. 3) The large pyroninophilic cells were markedly increased in the same area on 3 days after injection of the antigen and most of them were labeled with the injected thymidine-H³. 4) On 8 days after the injection of the antigen, the large pyroninophilic cells were markedly decreased in the extrafollicular area, while the plasma cells labeled with thymidine-H³ were found in the subepithelial region of the palatine tonsils. The above results suggest that the large pyroninophilic cells in the palatine tonsil could be differentiated into the plasma cells.