1. A photomicrographic method was devised for the measurement of the dimensions of red cells in rouleaux in plasma. With normal blood valid conclusions were drawn from 80 cell measurements with a high degree of confidence.
2. The mean corpuscular volume and the surface area were computed from these dimensions by formulae which treated the cell as a spheroid.
3. The dimensions, mean corpuscular volume and surface area of normal human red cells were as follows: 8.28 µ diameter, 1.71 µ thickness, 82 µ3 volume and 134 µ2 surface area.
4. The computed mean corpuscular volume was in agreement with the refractometric determination.