Considerable attention is currently focused on hemoglobins from lower mammals, such as the pig, for potential use in cell-free blood substitute preparations safe for use in humans. As the first step in the elucidation of structure-function relationships in porcine hemoglobin, we have determined the three-dimensional structure of aquomet porcine hemoglobin at 2.8 A resolution. Overall, the porcine hemoglobin tetramer is structurally similar to that of human oxyhemoglobin, and the r.m.s. deviation of all backbone atoms (minus five residues at the amino and carboxyl termini of each subunit) is 0.8 A. This similarity is not unexpected given that human and porcine hemoglobins exhibit 85% sequence identity. However, regions of subtle structural differences are implicated in subtle functional differences between the two proteins, such as the 20 to 25% inhibition of the alkaline Bohr effect and the accompanying reduction in oxygen-linked chloride binding observed for porcine hemoglobin. The structural similarity of these two mammalian hemoglobins also rationalizes the novel hybridization behavior of pig and human subunits in transgenic pigs expressing both porcine and human hemoglobins in porcine erythrocytes.