The acidic proline-rich proteins (Pr) showing genetic polymorphism were purified from human parotid salivas by gel filtration and ion exchange chromatography. Molecular weight determinations, amino acid composition analyses, and polypeptide mapping experiments indicate that the Pr 3 protein is a fragment of the Pr 1 protein. Studies of a parotid saliva factor capable of converting Pr 1 to Pr 3 and Pr 2 to Pr 4 indicate that Pr 3 and Pr 4 are generated from Pr 1 and Pr 2, respectively. Evidence suggests that the converting factor is a protease capable of posttranslationally cleaving Pr 1 and Pr 2, the primary or derived products of alleles Pr1 and Pr2.