Theoretical considerations are developed to account for the fact that different hemins have been obtained from the blood of apparently similar normal animals (oxen). Hemoglobin is a complex tribasic acid, 2 of the dissociated H atoms coming from car-boxyls of the globin and one from a carboxyl of the prosthetic group. Assuming that the 2 carboxyls of the prosthetic group are different, different hemoglobins, Aa and Ab, are possible, depending on whether carboxyl a or carboxyl b of the prosthetic group is in combination with globin. The splitting of hemin from hemoglobin in the H2SO4-methyl alcohol method of preparation involves methemoglobin as an intermediate step. Variation in the mechanism of this splitting gives different hemins. Also when a hemin is formed, one or both of the carboxyls of the prosthetic group react with the pyrrol N atoms to form betaine-like inner salts, which will differ in stability. Four isomeric mono-methyl hemins are therefore possible from each of the hemoglobins, Aa and Ab, a total of 8. On further methylation a total of 4 di-methyl hemins would result. The chemical properties of these isomers are discussed, and a number of hemin preparations are assigned structures in accordance with their properties.