Summary: Antigen stimulated cells leaving the lymph node by way of the efferent lymph are predominantly low density cells. 80–90% of the antibody producing cells accumulating in the density region 1·020 to 1·042 g./cc. Although 85% of the cells in this fraction were classified as antigen stimulated cells on morphological grounds, only 8% of them were synthesizing detectable antibody at the time of collection. Density gradient fractionation provides an effective means for separating these cells from the unstimulated cells in the lymph stream. Plaque‐forming cells isolated from the lymph node cover a much wider density range; some 70% of these cells have a density greater than 1·042 g./cc. Morphological analysis indicates that the low density cells produced in the lymph node by antigenic stimulation can readily leave the rode via the efferent lymph, while the high density cells, which include the plasma cells, tend to remain associated with the lymph node.