Certain blood characteristics were studied in normal men living at an altitude of 14,890 feet, with a barometric pressure of 410 mm. Hg. The following conclusions are indicated: (1) Average erythrocyte count is about a million per cu. mm. over normal sea level values, but in many individuals red cell count was normal; (2) the average percent of Hb in the blood is almost the same as the normal at sea level; (3) hemato-crit determinations show a greater increase than do the red cell counts; (4) the concentration of Hb in the red cells is low; (5) the foregoing facts seem to suggest that adaptation to high altitude does not involve primarily increases in red cell count and in amount of Hb, as has often been said, but rather a close correlation between cell number, volume, and Hb, and in the existence of a larger surface area in a given volume of blood; (6) viscosity of blood is increased; (7) red cells show marked increase in resistance to hemolysis; (8) a positive, indirect Van den Bergh reaction was obtained in most cases; (9) blood coagulation time increased slightly; (10) leucocyte count and leucocyte formula are nearly normal.