Most living cells derived from solid tissues require an adhering surface to live in vitro conditions. A good understanding of the relationships between the behavior of cells and the physicochemical properties of substrates such as the surface free energy, the surface polarity, the presence of functional groups and surface charges is of prime importance for the optimization of adhesion, spreading and proliferation of cells. Polystyrene and treated polystyrene surfaces were characterized by determining their surface free energies using wettability measurements. The knowledge of the surface properties of the culture substrates provides a good view of the influence of the substrate properties on cell adhesion. However, this study shows that it is not directly the surface free energy of materials that controls cell adhesion but rather the interfacial free energy between the culture medium and the substrate. The interfacial free energy between the culture medium and the solid surface controls the adsorption of serum components that may inhibit or promote cell adhesion. One of the components inhibiting cell adhesion is serum albumin. The results indicate that the adsorption of serum albumin is related to the interfacial free energy between the culture medium and the substrate. Hydrophilic substrates, such as plasma treated polystyrene substrates, have a lower interfacial free energy with water than hydrophobic polystyrene leading to a lower adsorption of proteins inhibiting cell adhesion. In addition, it is observed that there is a competition between proteins inhibiting (serum albumin) and proteins promoting cell adhesion (such as fibronectin). (C) Koninklijke Brill NV, Leiden, 2010