Background : Hemoglobin (Hb) has peroxidase (POD)-like activity. We found that the addition of albumin to human Hb purified from venous erythrocytes increased its POD-like activity. Methods : We treated human Hb with a Blue-Toyopearl column of immobilized albumin, compared the treated Hb and native Hb for POD-like activity, the level of Hb-bound 2,3-diphosphoglycerate (2,3-DPG), and the absorption spectrum, and found that treatment with albumin released 2,3-DPG from Hb, resulting in a tense-relaxed (T-R) conversion and increased POD-like activity. Results : The addition of human, mouse, rat, or bovine albumin to human Hb increased its POD-like activity. Addition of human albumin caused the highest increase, followed by that of mouse, rat, and bovine albumin in order. Addition of rabbit or guinea pig albumin caused little or no increase in the POD-like activity of Hb. Analysis of the distribution of methionine residues in the albumins of these animals showed that human, mouse, and rat albumins have a methionine residue at position 8 in loop 3 of domain I, and human and mouse albumins have an additional methionine residue in domain I. Bovine albumin has no methionine residue at position 8 in loop 3 of domain I, but has two methionine residues at other positions in domain I. Mouse and rat albumins have no methionine residues in domain I. Conclusions : These results suggest that the methionine residue at position 8 in loop 3 of domain I is most closely involved in the T-R conversion of human Hb. The addition of 2,3-DPG to albumin or selective oxidation of the methionine residues of albumin lessened the increase in POD-like activity when albumin was added to Hb, providing supporting evidence that the methionine residue of albumin is involved in the T-R conversion. The methionine residue of albumin may have a very important role in the degradation of Hb released from erythrocytes in blood vessels. Clin Chem Lab Med 2006;44:1131–6.