A polymorphic acidic protein (Pa) has been isolated from human parotid saliva by the use of ion-exchange and gel filtration chromatography. Following these purification procedures, analytical anionic polyacrylamide disc gel electrophoresis revealed a single stainable band. Amino acid analysis demonstrated a protein particularly rich in proline, glutamic acid, and glycine, but with reduced amounts of threonine and no tyrosine. Only a very small percentage of carbohydrate was detected. Isoelectric focusing at pH 3-10 verified the acidic character of this protein with an isoelectric point in the range pH 3.9-4.5. Other salivary proteins called Pa-II, possibly related physiologically and genetically to the Pr system, were also partially purified and studied. Differences were noted between Pa and Pa-II proteins in molecular size and amino acid composition.