Erythrocruorin (from Greek eruthros "red" + Latin cruor "blood"), and the similar chlorocruorin (from Greek khlōros "green" + Latin cruor "blood"), are large oxygen-carrying hemeprotein complexes, which have a molecular mass greater than 3.5 million daltons.

[1] Both are sometimes called giant hemoglobin or hexagonal bilayer haemoglobin.

Both contain many 16–17 kDa myoglobin-like subunits arranged in a giant complex of over a hundred subunits with interlinking proteins as well with a total weight exceeding 3600 kDa.

[6] The exact stoichiometric ratios and arrangement is unknown, but is thought to resemble that of erythocrorins.

[7][8][9] This enormous macromolecule is typically found free floating in the plasma, and not contained within red blood cells.