The av. lymphocyte output by way of the thoracic duct in the cat is 35.5 X 106 lymphocytes/kg. of body wt./hr. This figure is sufficiently great to account for the replacement of all the lymphocytes in the blood of a cat every 10-12 hrs. The number of lymphocytes in the blood of the cat diminishes appreciably during a 6 hr. period of thoracic duct cannulation. However, this diminution can be shown to be due, only in part, to loss of lymphocytes in lymph passing out in the fistula since a comparable diminution occurs in control animals on whom the same operative procedures are carried out except for the actual cannulation. The avg. thoracic duct lymph flow in the cat is 5.02 ml./hr. Differential counts of thoracic duct lymph yield the following results: eosinophiles 0.24% ; neutrophiles 1.12%; lymphocytes 98.56%, unclassified cells 0.12%. A few red blood cells are constantly present.