We report the isolation and the functional characterization of alpha and beta chains from pig (Sus scropha domesticus) haemoglobin, as well as of the pig-human hybrid haemoglobins, alpha2(h)beta2(p) and alpha2(p)beta2(h) (i.e. Circe's haemoglobins), obtained by mixing the purified alpha and beta pig chains respectively with the corresponding partner human chains. Their functional properties have been compared with those of both parental haemoglobins in order to obtain information on the role of the different subunits and of their inter-relationships, both at the structural and functional levels. The results indicate that the functional properties of both hybrids are closer to those of the parental haemoglobin that provides the beta chains, confirming the major role of the beta chains in determining the oxygen affinity and the modulation mechanisms of the tetrameric molecule. This is supported by the thermodynamic properties, since the very low DeltaH of oxygen binding that characterizes pig haemoglobin and the alpha2(h)beta2(p) hybrid haemoglobin may be taken as the reflection of specific structural properties of pig beta chain.