The complete amino acid sequence of a basic proline-rich protein, IB-9, from human parotid saliva was determined by automated Edman degradation of peptides obtained by enzymatic cleavage of the intact protein with clostripain. The protein was digested with papain and elastase to obtain overlapping peptides. Automated Edman degradation of the intact protein was also performed. The protein consists of 61 amino acids, of which 26 are proline residues. The partial sequence of another human parotid basic proline-rich protein, IB-6, was also obtained. With one exception the first 54 residues of the two proteins are identical. An exceptional degree of internal reiteration occurs in both molecules, including several repeated sequences of 12-14 amino acids. The proteins show a high degree of homology with the C-terminal portion of the salivary acidic proline-rich protein C.