This thesis reports investigations of the protein molecule myoglobin by means of electron paramagnetic resonance and supplemented by optical absorption measurements. Certain ions can be attached to the sixth coordination point of the iron atom in the haem site of the myoglobin molecule. Attachment of these ions causes the iron ion to be in either the ferric or ferrous state. Several ferric types (or derivatives) of myoglobin are studied at microwave frequencies of 35 GHz and 70 GHz.