In this paper, streptococcal cell surfaces are chemically modified, and the effects of the modification on cell surface hydrophobicity and charge, together with adhesion to hexadecane are determined. Acetic and succinic anhydrides, neutralizing or converting ammonium groups into negatively charged groups, respectively, had little influence on the water contact angle of the Streptococcus salivarius HB, but acetic anhydride modification reduced the water contact angle of the slightly more hydrophobic Streptococcus oralis 34 from 74 to 58 degrees. This reduction in water contact angle was accompanied by a more negatively charged cell surface, as inferred from particulate microelectrophoresis and a decrease in the amount of nitrogen-rich surface groups, detected by X-ray photoelectron spectroscopy. Carbodiimide followed by ethanolamine or ethylenediamine, compensating negatively charged carboxylate groups, slightly increased the water contact angles on the hydrophobic S. oralis 34, but strongly increased the water contact angle on S. salivarius HB by 32 degrees on average. Simultaneously, both strains became positively charged over the pH range from 2 to 9 and more nitrogen-rich groups were detected on the cell surfaces at the expense of oxygen-rich groups. Adhesion to hexadecane of the streptococcal strains was hardly affected by anhydride modifications, but both strains adhered better to hexadecane after ethanolamine or ethylenediamine modifications, especially at elevated pH values due to electrostatic attraction between the negatively charged hexadecane and the positively charged cell surfaces. This study shows that physico-chemical cell surface properties of streptococci can be chemically modified to stimulate their adhesion to surfaces.