Using tritiated thymidine as a cell label, the rate of disappearance of plasma cells from stimulated and unstimulated popliteal lymph nodes was followed for a period of 6 months in rats. In stimulated lymph nodes there was a rapid early decrease in plasma cell numbers, but a small percentage of labeled plasma cells persisted through the 6-month time point. In the unstimulated nodes, fewer plasma cells were initially labeled, but some of these also persisted throughout the span of the experiment. These cells could not have been formed from any other cell type during the course of the experiment, with the exception of direct transformation of small lymphocytes, because precursor and intermediate types of cells were not sufficiently highly labeled after the first 3 weeks.

Some long-lived plasma cells contain, and probably produce, antibody for at least 3 months, the longest time point tested.

Small lymphocytes also persisted within the popliteal nodes throughout the time of the experiment in both stimulated and unstimulated rats. The possibility that this fact may be important in secondary immunologic reactions is discussed.

The use of the footpad as the injection site for tritiated thymidine gave a degree of localization of label in the popliteal lymph node cells that allowed some observations on the migration of cells from these nodes. In this respect small lymphocytes were found to migrate to the submucosa of the ileum, and in lesser numbers to the spleen and thymus and probably the mesenteric lymph node. Throughout the course of the experiment, only two labeled plasma cells were found outside the popliteal nodes, both in the submucosa of the ileum.