Hemoproteins may present several functions due to their prosthetic groups. After a long time, well-studied proteins such as myoglobin have surprised us with new functions. Myoglobin is a hemoprotein which has some well described and unexpected functions within the organism. Oxidase activity in standard myoglobins has been described and this activity was attributed to a covalent linkage between heme and some amino acid residues such as histidine, when myoglobins are treated with alkyl halides, and tyrosine, and when myoglobins are treated with H 2 O 2 . We have found that the oxidase activity, due to H 2 O 2 treatment, can appear in different myoglobins, which presents no key residue, such as Tyr 103, for the oxidase activity previously described in the literature.