A basic proline-rich peptide, P-C, was isolated from human whole saliva and its amino acid sequence was determined to be Gly-Arg-Pro-Gln-Gly-Pro-Pro-Gln-Gln-Gly-Gly-His-Gln-Gln-Gly-Pro-Pro-Pro-Pro-Pro-Pro-Gly-Lys-Pro-Gln-Gly-Pro-Pro-Pro-Gln-Gly-Gly-Arg-Pro-Gln-Gly-Pro-Pro-Gln-Gly-Gln-Ser-Pro-Gln. P-C was also found to be present in stimulated parotid saliva. The sequence determined appears to correspond to the C-terminal 44 residues of a proline-rich phosphoprotein, Protein C, the partial sequence of which has been elucidated, and suggests that P-C might be derived from Protein C.