Immune responses which give rise to the synthesis of antibodies of restricted heterogeneity can be reproducibly induced in rabbits and mice by streptococcal polysaccharide antigens (1,2). While these reports have demonstrated clonotype restriction in the immune serum of rabbits and mice, they give little information with respect to the clontype restriction in a single reaction site of organized lymphoid tissue, for example a single lymph node. The kinetics of immune responses in lymph nodes in situ have been studied in sheep using a variety of antigens (3-6), and a substantial body of information is available on the changes in cell output, cell type, and the number of antibody-secreting cells which occur within the efferent lymph of antigen-stimulated lymph nodes. However, there is no information with respect to the amount and the heterogeneity of antibody that is secreted by the lymph node or cells within the efferent lymph. In the present report we have examined the temporal sequence of clonal restriction in the efferent lymph of individual sheep popliteal lymph nodes undergoing an immune response to the streptococcal group A-variant polysaccharide (Av-CHO).