1. Experiments designed to study, in the rabbit, the passage of foreign proteins from the blood stream into synovial fluid and to compare such passage with that taking place into the aqueous humor, spinal fluid, and urine are described.
2. Crystalline egg albumin and horse serum proteins regularly appeared in the knee joints within short periods of time following their intravenous injection.
3. These proteins also appeared promptly in the aqueous humor but in lower concentrations. In the spinal fluid they appeared only rarely and in minimal amounts.
4. Crystalline egg albumin was readily eliminated from the body via the urine. It was also removed rapidly from the knee joint and anterior chamber of the eye.
5. Horse serum proteins appeared only occasionally in the urine. Their concentration in the blood serum remained relatively high for several days.
Their increased concentration in the joint fluids in the longer experiments indicates that the rate of entrance exceeded the rate of removal.
6. Foreign proteins of the type employed were all found in the joint fluids in higher concentrations than they were in the other body fluids examined.
7. The possible significance of this study with respect to normal joint physiology and to certain abnormal joint conditions has been commented upon.