The acquired enamel pellicle is an oral, fluid-derived protein layer that forms on the tooth surface. It is a biologically and clinically important integument that protects teeth against enamel demineralization, and abrasion. Tooth surfaces are exposed to different proteinaceous microenvironments depending on the enamel location. For instance, tooth surfaces close to the gingival sulcus contact serum proteins that emanate via this sulcus, which may impact pellicle composition locally. The aims of this study were to define the major salivary and serum components that adsorb to hydroxyapatite, to study competition among them, and to obtain preliminary evidence in an in vivo saliva/serum pellicle model. Hydroxyapatite powder was incubated with saliva and serum, and the proteins that adsorbed were identified by mass spectrometry. To study competition, saliva and serum proteins were labeled with CyDyes, mixed in various proportions, and incubated with hydroxyapatite. In vivo competition was assessed using a split-mouth design, with half the buccal tooth surfaces coated with serum and the other half with saliva. After exposure to the oral environment for 0 min, 30 min and 2 h, the pellicles were analyzed by SDS-PAGE. In pure saliva- or serum-derived pellicles, 82 and 84 proteins were identified, respectively. When present concomitantly, salivary protein adsorbers effectively competed with serum protein adsorbers for the hydroxyapatite surface. Specifically, acidic proline-rich protein, cystatin, statherin and protein S100-A9 proteins competed off apolipoproteins, complement C4-A, haptoglobin, transthyretin and serotransferrin. In vivo evidence further supported the replacement of serum proteins by salivary proteins. In conclusion, although significant numbers of serum proteins emanate from the gingival sulcus, their ability to participate in dental pellicle formation is likely reduced in the presence of strong salivary protein adsorbers. The functional properties of the acquired enamel pellicle will therefore be mostly dictated by the salivary component.