Bovine caseinate, derivatives of its glycosylated moiety [caseinoglycomacropeptide (CGP)], and caseinophosphopeptides were evaluated as inhibitors of adhesion of oral bacteria to saliva-coated hydroxyapatite beads (S-HA). All milk casein-derived components behaved as potent inhibitors of Streptococcus sanguis OMZ 9 and Streptococcus sobrinus OMZ 176 adhesion to S-HA, whereas neither bovine serum albumin nor polyethyleneglycol were able to interfere with the adhesion of these strains. By contrast, none of the molecular species tested was able to inhibit the attachment of Actinomyces viscosus Ny 1 to S-HA. On the other hand, casein derivatives were shown to displace human serum albumin from S-HA beads. They were also able to bind to the bacterial cell surface of all strains examined. Collectively, these findings suggest that interactions between acidic casein-derived milk components and the biological surfaces involved in bacterial adhesion to S-HA result in an inhibitory effect that is selective for the oral streptococci examined.