The kinetics of the lymphoblast response in mice during the course of a primary infection with Hymenolepis nana was measured by the in vivo uptake of 125IUdR. The response was most marked in tissues local to the site of infection, involving to nodes draining the small intestine but not other areas, e.g., inguinal lymph nodes. A close correlation between these responses and the course of inflection was observed. Uptake of 125IUdR was greatest in the mesenteric lymph node (MLN) but the peak reached in this organ was later than that in Peyer's patches (PP), small intestine (SI) and spleen (S). The increase in lymphoblast activity of the MLN was similar with Trichinella spiralis; no significant blast cell response to inflection with H. diminuta was found till day 9 after injection, the results being similar to those obtained when H. nana inflections were established using cysticercoids rather than eggs. It has been shown that the increase in lymphoblast activity was closely correlated with the presence of cells which are most effective in adoptive transfer immunity. A dose-dependent effect was detected in blast cell activity of MLN in different inflection levels with T. spiralis and H. nana.