The Collaborative International Dictionary of English v.0.48:

Methaemoglobin \Met`haem*o*glo"bin\ (? or ?), n. [Pref. met- +
   haemoglobin.] (Physiol. Chem.)
   A stable crystalline compound obtained by the decomposition
   of hemoglobin. It is found in old blood stains.
   [1913 Webster] methadon