In an attempt to localize an antigen in a single lymph node, sheep erythrocytes were injected directly into a single popliteal lymph node of an adult rabbit by way of an afferent lymphatic, after blocking all its lymphatics. The condition of complete obstruction of the lymphatic drainage through the injected popliteal node was maintained for a period as long as 14 days. Evidence was obtained to indicate that the injected antigen, if given in appropriate low doses, was localized completely in the injected popliteal node. Subsequent to antigenic stimulation, a primay immune response developed, with proliferative reaction of antibody-producing cells which was also confined to the stimulated popliteal node almost completely. Despite complete obstruction of the lymphatic pathways of the stimulated popliteal node, the serum antibody titers were considerably raised, though to a lesser extent than in the control animals in which the lymphatic drainage of the antigen-stimulated popliteal node was left intact. The possible ways of transport of antibody from the antigen-stimulated lymph node to the general circulation were discussed.