1. Phosphate exchange in red cells and plasma was studied in vitro using P32 in the form of sodium phosphate as a tracer.
2. No phosphate was added other than the isotopic preparation which was of high specific activity.
3. Inorganic phosphate exchanged freely between the plasma and the erythrocytes at 37.5 C. in a period of four hours. Minimal transfer occurred at 7 C.
4. Most of the added P32 which passed into the erythrocytes during this time remained in the inorganic fraction, less than 15 per cent being found in the organic acid soluble fraction.
5. The specific activity of the inorganic phosphate of the erythrocytes was equal to or greater than that obtaining for the inorganic phosphate of the plasma at the end of the four hour incubation period at 37.5 C.