Saliva is a body fluid of a unique composition devoted to protect the mouth cavity and the digestive tract. Our high performance liquid chromatography (HPLC)-electrospray ionization-MS analysis of the acidic soluble fraction of saliva from preterm human newborn surprisingly revealed more than 40 protein masses often undetected in adult saliva. We were able to identify the following proteins: stefin A and stefin B, S100A7 (two isoforms), S100A8, S100A9 (four isoforms), S100A11, S100A12, small proline-rich protein 3 (two isoforms), lysozyme C, thymosins beta(4) and beta(10), antileukoproteinase, histone H1c, and alpha and gamma globins. The average mass value reported in international data banks was often incongruent with our experimental results mostly because of post-translational modifications of the proteins, e.g. acetylation of the N-terminal residue. A quantitative label-free MS analysis showed protein levels altered in relation to the postconceptional age and suggested coordinate and hierarchical functions for these proteins during development. In summary, this study shows for the first time that analysis of these proteins in saliva of preterm newborns might represent a noninvasive way to obtain precious information of the molecular mechanisms of development of human fetal oral structures. Molecular & Cellular Proteomics 10: 10.1074/mcp.M110.003467, 1-14, 2011.