The influence of salivary pellicles on the demineralization of human dental enamel by 1% citric acid was studied. The pellicles were formed on embedded human tooth surfaces incubated in vitro with various solutions for periods of up to 6 days. Pellicles induced by unstimulated whole saliva gave an approximately 45% and those from parotid saliva a 25% inhibition of demineralization. On the other hand, a pellicle formed from mixed submandibular and sublingual (SM-SL) saliva in 30 min gave a retardation in lesion formation of at least 40% and after a 60-min exposure to SM-SL saliva, the lesion formation was completely prevented. In contrast, when the mucins were removed from SM-SL saliva by ultracentrifugation, protection against demineralization by the pellicle formed on exposure to the supernatant was only 30%. The effect of pellicles obtained by isolated salivary mucins was also studied. Human whole salivary mucins (HWSM) were isolated from both human SM-SL saliva and whole saliva of two individuals with blood group A. The mucins contained 18% protein, 72% carbohydrate, 1.4% sulfate and 0.14% phosphate. The major components of the protein moiety were threonine (14.7%), valine (12.6%) and serine (10.8%). The molar ratio of the sugar residues was: galactose: N-acetylglucosamine: N-acetylgalactose: fucose: N-acetylneuraminic acid = 6:4.5:4:7:1. A pellicle formed by a 3-day exposure to salivary mucins in vitro gave 100% protection of the tooth surface against demineralization by citric acid. These data suggest that the mucins in human saliva contribute to a large extent to its protective effect against acidic attacks on the tooth surface.