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 µm. Each series consisted of four substrates with varying surface free energies: pure aluminum oxide, 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 measurement 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°. One silanization procedure gave a surface of intermediate hydrophobicity, with a y-tot of 47.5 mN/m; the other gave a strongly hydrophobic surface of y-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_x001F_porous substrate and that the PEG treatment, as well as the two sil aniz ation pro cedures, resulted in a decreas e in bacterial adhesion, b oth to the controls treated with aqueous buffer and to surfaces treated with fresh saliva.