Summary: The cells of the popliteal lymph nodes of rats were labeled with tritiated thymidine for 4 days after primary or secondary injections of OT or after a secondary injection of typhoid vaccine. Thiry-one days after the last dose of tymidine, rats of each group were challenged with typhoid vaccine. Increased numbers of heavily-labeled large lymphocytes and mitotic figures were found in the popliteal nodes for 2 days after challenge with typhoid vaccine regardless of the antigen given prior to labeling. This in situ reaction to typhoid vaccine was demonstrated as long as 7 months after labeling. Reasons indicating that these large lymphoid cells were derived from labeled, long-lived small lymphocytes are discussed. We believe these results confirm the validity of the method of following tritiated thymidine-labeled lymphocytes for long periods of time.