Interactions between amphiphilic alpha-helical human apolipoprotein CI (APO CI) adsorbed on hydrophilic and on hydrophobic surfaces were studied using an interferometric surface force apparatus in an effort to understand the surface conformation and the binding activity of this protein. We used mica as the hydrophilic substrate and polymerized octadecyltriethoxysilane (OTE)-covered mica as the hydrophobic substrate. The OTE monolayer and the OTE Langmuir-Blodgett film were studied using Brewster angle microscopy and atomic force microscopy, respectively. We found that interaction forces between layers of APO CI adsorbed on hydrophilic and on hydrophobic surfaces are mainly due to electrostatic double-layer forces at large surface distances and to steric repulsive forces at small distances. In some cases, no force was measured prior to finding a steric wall, suggesting that a complete neutralization of the surface charge was achieved by the protein adsorption. Protein layer thickness values allow us to give an image of the organization and conformation of the APO CI protein on surfaces. The adhesion obtained in both kinds of surfaces indicates that the interaction between the hydrophobic sides of the APO CI proteins is stronger than that between the hydrophilic sides of the protein.