The amount of adenosine deaminase activity in free-floating lymphoid cells obtained from the efferent duct of the popliteal node of sheep has been shown to increase after the node was antigenically stimulated by chicken red cells. The increase in cellular enzyme content coincided with the appearance of plasma cells in the lymph and the presence of intra-cellular antibody. The activity of the free enzyme was found to be higher in intermediate lymph than in peripheral lymph or blood plasma.