Two types of experiment using local labeling of lymph nodes with FITC showed that lymphocytes emigrate from lymph nodes, predominantly in blood in the pig and in efferent lymph in the sheep. In the first type of experiment with the pig, few cells emigrated via the lymph, while the number of labelled cells in the blood increased progressively and the indices in mesenteric blood were always higher than in jugular blood in simultaneously-drawn samples. However, in the sheep, when efferent lymph flowed freely, very low numbers emerged in blood and continuing large numbers of lymphocytes emerged in efferent lymph. In the second type of experiment carried out wholely under anaesthetic on mesenteric lymph nodes in pigs and sheep, and on superficial inguinal lymph nodes in pigs, the lymph node was isolated, the lymph and venous drainage collected and only the arterial supply maintained. Large numbers of FITC+ lymphocytes emigrated via the vein in pigs with either node cannulation (i.e. up to 7% blood lymphocytes were labelled with an emigration rate of approximately 10(8) cells/hr) but in sheep, while lymph contained approximately 30-80% labelled cells and the emigration rate was also approximately 10(8) cells/hr, the mesenteric blood contained very few labelled cells (approximately 0.2%, giving a mean venous emigration rate of 2.7 X 10(6)/hr). Study of the type of lymphocytes emerging from labelled pig lymph nodes and spleen during the phase of major emigration showed that sIg+ B and E rosette-forming T cells, but almost no Null cells, are involved.