The binding of sodium n-dodecyl sulphate (SDS) to pigeon and bovine haemoglobin has been studied at pH 6.4 and 25 degrees C by equilibrium dialysis and titration microcalorimetry techniques. The thermodynamic parameters of these interactions have been determined by application of the Wyman binding potential concept and interpreted from a structural viewpoint. A comparison of the results shows that bovine haemoglobin has more thermodynamic stability than pigeon haemoglobin. From this, a reason for the high oxygen affinity of avian haemoglobin is proposed.