The ferric form of reindeer hemoglobin (Rangifer tarandus tarandus) has been crystallized in an orthorhombic crystalline form from polyethylene glycol solutions, at pH 8.2. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit cell edges a = 84.2 A, b = 59.9 A, c = 119.5 A; one hemoglobin tetramer is contained in the asymmetric unit. The crystals diffract X-rays to a limit spacing of 3.0 A. Inspection of amino acid sequences in the N-terminal region of beta-chains, and analysis of hemoglobin three-dimensional models, allows one to rationalize, on a molecular basis, the reduced O2 affinity and the decreased effect of organic phosphates observed in ruminant hemoglobins. By analogy, the analysis is extended to birds and reptiles, whose hemoglobin beta-chains display, as in ruminants, the deletion of the N-terminal residue and a methionine at the NA2 position.