Molecular dynamics simulations were performed to determine the force field of attraction between Leu-enkephalin and a model polyethylene surface. Four separate rotational orientations of the polypeptide were simulated. During the simulations the surface atoms were held static, but the water atoms were dynamic. Some simulations were studied employing restricted dynamics of the polypeptide in which only the five backbone ct-carbon atoms held immobile. For an orientation with oxygen atoms toward the PE surface the force between the enkephalin and the surface was repulsive and was approaching zero as the separation reached 8 Angstrom. The maximum repulsive force reached 8 kcal/mole/Angstrom at 2.7 Angstrom separation. For an orientation with hydrophobic groups toward the PE, the force was attractive with a minimum in the force field of 4 kcal/mole/Angstrom at 2.3 Angstrom. Two other orientations were also investigated. The results of the MD simulations indicated that the force between enkephalin and PE in the region 0 < z < 9.5 Angstrom was mostly a function of the separation distance and the orientation angle of enkephalin with respect to the surface.