These data prove that abnormally high oxygen unsaturation of the blood is a cause of cyanosis. The fact that the lowest value of oxygen unsaturation (in the venous blood) associated with cyanosis is about 8 volumes per cent seems to indicate that this amount of reduced hemoglobin is the lowest capable of producing a cyanotic color. We shall later discuss this point more in detail. Table III shows furthermore that in spite of the fact that cyanosis is due to abnormally high oxygen unsaturation, no proportionality exists between the intensity of the blue color and the amount of reduced hemoglobin. This may in small part be due to individual peculiarities of the skin and subcutaneous tissue, which are known to influence in anemia the relation between paleness of the skin and the decrease in hemoglobin. We shall, however, in the succeeding paper show that the main cause of the disproportionality between cyanosis and venous oxygen unsaturation is found in another factor, the recognition of which throws a clearer light on the pathogenesis of cyanosis, and explains why we may find values of oxygen unsaturation as high as 13 volumes per cent in non-cyanotic individuals.