1. Red cell volume and plasma volume were measured by the radioactive sodium chromate and the chromic chloride method respectively in 37 patients.
2. The body hematocrit/venous hematocrit (BH/VH) ratio was found to be constant in normal subjects and in patients in whom there was neither splenomegaly nor edema. The ratio was 0.896 ± 0.039.
3. In splenectomized patients the BH/VH ratio was normal indicating that in man, in normal conditions, the spleen does not have the function of a significant blood reservoir with raised cell/plasma ratio.
4. In patients with splenomegaly the BH/VH ratio was found to be significantly increased in direct proportion to the degree of splenic enlargement, indicating an increased concentration of erythrocytes in the enlarged spleen.
5. In the presence of extravascular fluid retention caused by prolonged prednisone treatment the BH/VH ratio was lower than normal.
6. It was concluded that in patients with splenomegaly or edema measurement of either red cell volume or plasma volume alone in conjunction with the "corrected" venous hematocrit does not permit valid assessment of the non-measured parameter nor of the total blood volume.