During determination of the oxygen capacity of blood by the Van Slyke method (1), the following adaptations were made in order to reduce errors: (1) Blood was equilibrated with water-saturated air in a siliconized tonometer. (2) The determination of the oxygen content of the sample was carried out immediately after equilibration. (3) Triton was used instead of saponin in the ferricyanide reagent. (4) Special care was taken in the preparation and storage of the dithionite reagent, which should be prepared fresh daily and stored in the dark at 4°. The hemoglobin content and oxygen capacity, measured by the modified technic, were compared in a series of 36 samples of human blood.