Microscale assays of protein, glucose, lactate (LA), and the total lactic dehydrogenase (LDH) activity in samples of free interstitial fluid from solid unicentric tumor transplants in mice are reported. The values have been compared with those found in plasma and intraperitoneal fluid from normal and tumor-bearing mice. The tumor fluids contained greater amounts of protein, LA, and LDH than did normal interstitial fluid, even when this was obtained from the same tumor-bearing mice. Highest

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... s were found in the central fluid of young tumors. The glucose content of tumor fluids was considerably lower than that of normal intraperitoneal fluid or plasma. It is concluded that the composition of the interstitial fluid of solid tumors is very different from that of normal interstitial fluid. The reasons for this are discussed with reference to the metabolism and permeability of tumor cells, and to the vascular sup ply and interstitial transport facilities in tumors.