1) The Soret region absorption spectrum of erythrocruorin (ERC) obtained from Glossoscolex paulistus, shows that oxy-ERC has a maximum absorption peak at 416 nm while the deoxy-ERC from has a maximum at 427 nm. 2) In the presence of a specific antiserum (anti-ERC) and of anti-ERC immunoglobulin G raised in rabbits, there is a deviation to low wavelengths in the maximum absorption peak of deoxy-ERC while for the oxy form a red-shift is noticed. These shifts accompanied an increased affinity of the hemeprotein for oxygen, possibly because of changes in the overall macromolecular conformation. 3) A decrease in the oxygen affinity of erythrocruorin is observed when large amounts of non-specific serum are used. The same effect is observed in the presence of serum albumin, probably as a result of non-specific binding between the albumin and erythrocruorin. 4) The fluorimetric titration of erythrocruorin with anti-ERC Fab fragments results in a decrease in the intrinsic tryptophan fluorescence of the hemeprotein, a response indicative of a modification in the ERC's quaternary structure.41497-509