Trypan blue in saline solution was injected into a living normal knee joint, one hour before amputation. During this time 99 per cent. was absorbed, and microscopic examination showed that the dye had stained only lightly the synovial and subsynovial tissues. In general, the synovial cells were devoid of dye granules, but these were visible in the macrophages, the fibroblasts, and the monocytes.
The same amount of trypan blue was injected into the ankle joint after amputation. After five hours, 88 per cent. had been absorbed ; but much of this remained fixed in the synovial membrane, and both cells and matrix showed intense staining to a depth of 2 millimeters.
It is concluded that trypan blue is absorbed rapidly from the normal living knee joint, mainly by diffusion into the blood and, to a much less degree, by way of the lymph and by phagocytosis. Circulation to the joint is shown to be an important factor.