Evidence for the amino acid sequence of some peptides formed by the action of trypsin on the β-polypeptide chain of horse hemoglobin is presented. By analogy with the amino acid sequence of the β-chain of human hemoglobin, these peptides cover positions 1 to 82 and 117 to 146 of the horse β-chain. Twenty-one differences between the human and horse β-chain sequence are found in these regions.