The numbers of the PFC in the thoracic duct and lymph node of the immunized rabbit were counted by the hemolytic plaque method and the morphologic features of the PFC in the lymph node were simultaneously observed by phase contrast microscopy.

The number of the PFC in the lymph node maintained a relatively high level on the fourth to seventh days, and then rose to attain the maximum on the ninth to thirteenth days, and then decreased.

The number of the PFC in the thoracic duct reached the maximum on the fourth day after the immunization and then dropped rapidly. The mechanism was discussed.

From morphologic observations, the PFC were classified into lymphogonia, lymphoblast, basophilic mature lymphocyte and plasma cell. The lymphogonia and lymphoblast, which belonged to immature lymphocyte series, were frequent in the early stage and the plasma cell was increased in the later stage of the immunization. The basophilic mature lymphocyte constituted more than 55 per cent through the whole process and were most frequent of the plaque forming cells.

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