Human glandular salivary secretions contain several acidic proline-rich phosphoproteins (PRPs). These proteins have important biological functions related to providing a protective environment for the teeth, and appear to possess other activities associated with modulation of adhesion of bacteria to oral surfaces. These functions and activities depend on the primary structures of the PRPs. Previously determined amino acid sequences of two 150-residue molecules, PRP-1 and PRP-2, and two related 106-residue proteins, PRP-3 and PRP-4, indicated that residue 4 was Asn in PRP-1 and PRP-3, and Asp in PRP-2 and PRP-4, and position 50 was Asn in all four proteins. Recent data from cDNA sequence studies and further structural studies, however, showed that the previously proposed sequences cannot be completely correct. The present work has shown that the protein previously designated as PRP-1 actually consisted of two positional isomers, PIF-s, which has Asn and Asp at positions 4 and 50 respectively, and authentic PRP-1, which has the reverse arrangement. The same isomerism is present in the smaller proteins, PIF-f and PRP-3. Since the isomeric pairs have identical compositions and charges, their presence was not previously detected. Also, by using a more highly purified preparation, it has been found that position 50 in PRP-2 and PRP-4 is Asp, rather than Asn previously reported. These new findings for the six PRPs define their complete primary structures, which are now consistent with those proposed for PRP-1 and PIF-s from cDNA data, and are also consistent with the chromatographic and electrophoretic behaviours of the six PRPs and their derived peptides. These corrected structures are important for understanding the biological functions and activities of these unusual proteins.