To elucidate the molecular mechanism of the direct allosteric exchange of oxygen and hydrogen carbonate in the hemoglobins of crocodiles, the N-terminal sequence of the beta-chains of the crocodiles of the Nile (Crocodylus niloticus) and of the Mississippi (Alligator mississippiensis) was studied. The N-terminal end of the peptide is blocked. By mass spectrometry the N-terminal sequences of both species were found to be N-acetyl-alanyl-seryl-phenylalanine. These data explain the absence of hemoglobinphosphate interaction, the data are in good agreement with the stereochemistry postulated for allosteric exchange of oxygen and hydrogen carbonate in crocodilian hemoglobins.