IN RECENT years, analyses and electron microscope investigations have been made on the organic films which form in vivo on the enamel surfaces of erupted teeth (MECKEL, 1965; ARMSTRONG, 1966, 1967, 1968; ARMSTRONG and HAYWARD, 1968; LEACH and SAXTON, 1966; LEACH et al., 1967). These studies have indicated the probable involvement of salivary proteins in the formation of these structures. In-vitro observations, using disc-electrophoresis, have demonstrated the selective adsorption of specific salivary protein components on to enamel and synthetic hydroxyapatite crystal surfaces (HAY, 1967). With the development of a sensitive auto-analytic assay technique (ARMSTRONG, 1969), it has now proved possible to determine the amino acid composition of these in-vitro adsorbed proteins. Following the method described by HAY (1967), 10 ml supernatant aliquots from centrifuged whole human saliva samples, were stirred for 20 min with 50 mg quantities of a synthetic hydroxyapatite preparation (LEVIN, 1962). Subsequent disc electrophoresis (pH 4) showed the specific adsorption on the hydroxyapatite crystals of two major and one minor protein component, thus substantially confirming Hay’s observations. Amino-acid analysis of these adsorbed components, either after direct hydrolysis of the crystals or following elution by 0 *2M sodium phosphate buffer, pH 7.0, gave a characteristic composition as shown in Table 1 (columns 1 and 2 respectively). The most significant features of the composition of the salivary proteins selectively adsorbed by the hydroxyapatite crystals were the high proline content and the high glutamic acid and glycine levels. These three amino acids together constituted approximately half the total amino acid residues. Significant quantitites of hexosamine (O-5 per cent) were also present, implying a glycoprotein nature for the components. The very high proline levels have only previously been observed in foetal enamel proteins (EASTOE, 1963; LEVINE et al., 1967; FINCHAM, 1968), or in parotid salivary proteins (MANDEL, THOMPSON, ELLISON, 1965; ARMSTRONG, 1967) (columns 5 and 6 in Table 1). The latter feature suggested the probable involvement of parotid proteins in the deposition of these in-vitro adsorbed proteins. Stimulated parotid saliva was collected, using a Curby cup type Lashley cannula (CURBY, 1953), and added to the synthetic hydroxyapatite as described above. Subsequent disc electrophoresis indicated the specific uptake from parotid saliva of the same electrophoretic components adsorbed from whole saliva, but with a greater relative content of the faster