1. The degree of immunological identity of the hemopexin of 64 mammals and non-mammals is determined by double diffusion in agar and two radioimmunoassays, employing antisera produced with the human or the rabbit protein. 2. Antigenic determinants are shared by the Eutherian mammals but not by the non-Eutherian mammals and lower animals. 3. The hemopexins of man and apes appear to be identical, whereas those of Old World monkeys lack one antigenic determinant, and New World monkeys lack at least two antigenic determinants.