The four hemoglobin components (F1, F2, S1, and S2) of eel, Anguilla japonica, were analyzed for C-terminal amino acids by the hydrazinolysis method. Results obtained are summarized as follows: (1) In the cases of components F1 and F2, the valyl and the acetylated polypeptide chain have a His and an Arg residue, respectively, as C-terminus, while in the cases of components S1 and S2 both chains have exclusively an Arg residue per chain as Cterminus. (2) Together with the fact that components S1 and S2 are lacking in BOHR effect, whereas components F1 and F2 show large BOHR effect, the present results seem to give a new and strong support to the suggestion based on some indirect evidence with mammalian hemoglobins that the C-terminal His residues of β chains of hemoglobin are remarkably responsible for its BOHR effect and oxygen affinity.