Adhesion of bacteria to pellicle-coated tooth surfaces is the first step in biofilm formation on teeth. The aim of this study was to explore whether human milk prevents or promotes adhesion of cariogenic Streptococcus mutans to saliva-coated hydroxyapatite (HA) using an in vitro model system. S. mutans binding to HA coated with human parotid saliva (s-HA) or human milk was studied, in addition to binding inhibition to s-HA by human milk. S. mutans did not bind to HA coated with milk. S. mutans binding to s-HA was inhibited by milk from 15 (71 %) of 21 women, whereas milk from the remaining 6 mothers enhanced binding of S. mutans to s-HA. Inhibition of S. mutans binding correlated with bacterial binding to s-HA (r = 0.76). Human milk does not mediate adhesion of S. mutans to HA in vitro, but affects adhesion in an individually varying fashion. Phenotypic variations in milk and saliva glycosylation may explain the inhibitory capacity and possibly affect susceptibility to colonization by S. mutans in childhood. Copyright 2006 S. Karger AG, Basel.