The Bohr effect of the blood and the red cell hemolysates of adult and newborn humans, goats and kids, and sheep and lambs were determined and the physiological significance is discussed. Similar determinations were made on blood from an African elephant, yak, camel, Dybowski deer, and llama. The strong displacement to the right of the oxygen dissociation curve which occurs in kids and lambs during the first 5 days of life can be largely explained by a change in the pH value within the erythrocytes. When the oxygen affinity is expressed by the oxygen pressure necessary for half saturation (T50) at a constant cell pH, considerable differences exist between species.