Changes in the fine structure of the lymph nodes were studied after prolonged immunization with heterologous sera. Hypertrophy of the lymph node cortex and paracortex frequently occurred, as well as development of the zone lying next to the lymphatic sinuses, postcapillary venules, veins and arteries in the cortex, paracortex and medulla, which is mainly bursa-dependent. Worthy of note was the presence of germinative centers characterized by large ribosomes, rich cells and macrophages. Similar aspects were observed in splenectomized animals undergoing prolonged immunization. Particular aspects of immunologic activity in the lymph nodes were noticed in the animals which had received an immunosuppressive treatment during immunization. The significance of the morphological alterations in the fine structure of the lymph nodes is correlated with the serological findings.