Using equilibrium dialysis, isothermal titration calorimetry (ITC) and circular dichroism (CD), the interactions of perfluorooctanoic acid (PFOA) and lysozyme were investigated under normal human physiological conditions, i.e., at pH 4.40, 6.00 and 7.40 at 37 degrees C in 0.15 M electrolyte. A simple and rapid spectrophotometric method was developed for determining PFOA concentrations. Interactions between PFOA and lysozyme were found to result from non-specific non-covalent bonds-F/N and F/O affinity, ion-pair attraction, hydrogen bond, hydrophobic interaction and van der Waals force-and were affected by chemical adsorption to monolayers. The results indicated that binding of PFOA altered the secondary structure and activity of lysozyme. This work provides a useful experimental strategy for research into the enzyme toxicity of organic chemicals, e.g., food additives and organic contaminants, and it may help to elucidate the molecular toxicology of human health risks.