1. Data are presented showing that precipitin tests can be used for the detection of the proteins contained in egg white and horse serum in the blood stream and thoracic duct lymph following injection into the knee joints of normal dogs. The sera of rabbits immunized against the particular protein employed were used in doing the precipitin tests.

2. Egg white is removed only by way of the lymphatics, appearing more rapidly if the leg muscles are massaged. The removal is even greater from passively exercised joints.

3. Horse serum albumin is readily removed from a normal dog's knee joint by way of the lymphatics alone and with greater rapidity from a passively exercised joint. Horse serum globulin escapes from normal joints with great difficulty, if at all, and therefore does not readily gain entrance into the underlying lymphatic capillaries.

4. The molecular size of a protein readily gaining egress from a joint through the lymphatics is thus defined.

5. The relative sizes of the albumin and globulin molecules contained in horse serum are confirmed by in vivo experiments.

6. The lymphatic system is the essential apparatus for the removal of protein from joints, and any interference with this drainage should promote the formation of intraarticular effusion.

The data presented suggest that a simple clinical test for determining the efficiency of the lymphatics draining a single joint can be devised.

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