The interaction between natural lipopeptide [Glu(1), Asp(5)] surfactin-C15 (surfactin) and hemoglobin (Hb) has been studied. Surface tension Measurements show that the critical micelle concentration (cmc) Of surfactin increases from 1.54 x 10(-5) to 3.86 x 10(-5) mol/L with Hb. The UV spectra display that the effect Of surfactin oil Hb exhibits strong concentration-dependent fashion and the aquometHb convert to hemichrome at high surfactin concentration. Small-angle neutron scattering (SANS) and freeze-fracture transmission electron microscopy (FF-TEM) measurements show that surfactin result ill the formation of a fractal structure representing a "necklace model" of micelle-like clusters randomly distributed along the protein polypeptide chain at high surfactin concentration. Far-UV circular dichroism (CD) results confirmed that surfactin call disrupt the helical structure of protein at high concentrations, although the enhanced native-like behavior of protein by low concentration of surfactin was observed. The microenvironment change around Pile amino residues and disulfide bonds of Hb was obtained from near-UV CD spectra.