The literature provides all the data needed to calculate the ratio between the amount of haemoglobin and the total surface area of erythrocytes in 54 species of mammals ranging in body mass from 2.5 g to more than 1000 kg. Analysis shows that the concentration of haemoglobin (Hb; g%) does not defend on the body mass ofthe mammals studied. The number of erythrocytes in 1 mm of blood (RBC; 10 x mm ) is significantly lower, and the diameter of these cells significantly higher, among larger mammals as opposed to smaller ones. The result is that the total surface area of erythrocytes in 1 mm of blood (TSAE; mm x mm ) is significantly lower among larger mammals, while the Hb/TSAE ratio (pg x ^m- ) is significantly greater. These results point to the smaller size of erythrocytes of smaller mammals permitting much greater numbers to exist, thereby producing a greater TSAE and smaller Hb/TSAE ratio. The greater total surface area of red blood cells per unit volume of blood in small mammals can in turn be presumed to allow for full saturation of haemoglobin by oxygen, even where the period of contact between erythrocytes and air in the lungs is shorter than in their larger counterparts.