The effects of changes in groups attached to the periphery of the porphyrin ring of the heme of various [human, bovine, mouse, guinea pig, rabbit, opossum, chick, pigeon, bullfrog, carp, gar, torpedo ray and Glycera dibranchiata] Hb and [sperm whale, horse heart, dolphin] myoglobins on the environment experienced by the ligand, CO, were studied by observation of the chemical shift of the bound 13CO. The major interaction between bound ligand and substituents around the porphyrin is probably that transmitted electronically from substituent to ligand. The nature of the protein environment around the ligand and the interaction between the proximal histidine (F8) and the ligand (through the Fe atom) impose differences between subunits of Hb and between myoglobins and Hb which are largely, but not entirely, independent of these substituent effects. To assess the influence of protein structure on the chemical shifts of bound ligand, the shifts of 13CO bound to myoglobin and Hb from a wide range of species were also measured.