Adhesion of the oral bacterium Streptococcus sanguis was investigated on two series of surfaces, one solid, non-porous and one porous with a pore diameter centered around 0.2 mu m. Each series consisted of four substrates with varying surface free energies: pure aluminum oxide, poly(ethylene glycol) (PEG)-modified aluminum oxide, and two types of silanized aluminum oxide. The surface composition was analyzed by ESCA and the surface free energy and acid-base character were determined by contact angle measurements using water, formamide, and diiodomethane according to van Oss and Good. Contact angle measurements were conducted on non-porous substrates which had been subjected to the same silanization procedure as the porous ones. Untreated aluminum oxide and PEG-modified aluminum oxide were both hydrophilic and gave water contact angles of less than 10 degrees. One silanization procedure gave a surface of intermediate hydrophobicity, with a gamma(tot) of 47.5 mN/m; the other gave a strongly hydrophobic surface of gamma(tot) = 27.4 mN/m. Adhesion of S. sanguis was measured by using radiolabelled bacteria. It was demonstrated that more bacteria adhered to the porous than to the non-porous substrate and that the PEG treatment, as well as the two silanization procedures, resulted in a decrease in bacterial adhesion, in relation both to the controls treated with aqueous buffer and to surfaces treated with fresh saliva.