It has been generally assumed that mammals have blood with a greater temperature sensitivity than ectothermic organisms. Recent results have shown that in some species of mammals, Hb displays a value of overall oxygenation enthalpy (delta H) much less exothermic than that observed for most mammalian hemoglobins, including human adult Hb. In this respect, a very interesting case is represented by porcine blood which shows a modest effect of temperature, the temperature coefficient of its oxygen-dissociation curve being significantly lower than that of human blood. Here we report a detailed functional characterization of pig Hb, which, interpreted on the basis of the amino acid sequence of the alpha and beta chains of the molecule, sheds some light on the molecular basis of the phenomenon.