The concentration and output of lymphocytes in lymph from the popliteal node in sheep were varied by antigenic stimulation of the lymph node and by intravenous administration of bovine anti-lymphocyte serum (ALS). The 4-5-fold increase in output and concentration of lymphocytes in lymph following antigenic stimulation of the node was accompanied by a 25-35% increase in the concentration of total protein and albumin. The above changes were not observed in afferent lymph draining the site of antigen injection. There was a 75-80% decrease in concentration of lymphocytes in lymph from the unstimulated popliteal lymph node following the administration of ALS and this was accompanied by decreases of 10-35% in the concentration of albumin and immunoglobulin. Essentially the same changes were observed when ALS was administered 6 days after antigenic stimulation of the node. Administration of ALS to sheep with cannulated intestinal lymphatic ducts was followed by a 75-80% reduction in lymphocyte concentration and in decreases ranging from 12-50% in the concentration of albumin, IgM and IgG-2. It is suggested that a significant loss of plasma protein is associated with the migration of lymphocytes between the endothelial cells of the post-capillary venules in lymph nodes.