A hydrophobic tetraphenylporphyrinatoiron(II) derivative bearing a covalently bound axial imidazole [Fe(II)P] was efficiently and noncovalently bound into human serum albumin (HSA) up to an average of eight Fe(II)P molecules per HSA molecule. The aqueous solutions of the HSA-Fe(II)P complex provided a reversible and relatively stable oxygen adduct under physiological conditions (pH 7.4 and 37 degrees C). The half-life of the oxygen adduct (tau 1/2) was 1 h at 37 degrees C in an air atmosphere. With Fe(II)-TpivPP (the so-called "picket-fence heme") having no axial base, an oxygenated HSA-Fe(II)TpivPP complex was obtained using a 20-fold molar excess of 1,2-dimethylimidazole, but the tau 1/2 was very short (ca. 10 min at 37 degrees C). The oxygen affinity [P 1/2(O2)] and oxygen transporting efficiency (OTE) of HSA-Fe(II)P at 37 degrees C were 30 Torr and 22%, respectively. Furthermore, the oxygen-binding and dissociation rate constants (kon and koff) are extremely high in comparison with those of hemoglobin. The HSA molecule binding eight Fe(II)P molecules can transport about 3.4 mL/dL of oxygen under physiological conditions, corresponding to about 60% of the oxygen transporting amount of human blood.