A new polymorphic acidic proline-rich protein (As) was found in human parotid saliva by SDS and basic polyacrylamide gel electrophoresis. The phenotypic relationships and family studies support the hypothesis that the As protein is another allelic product of the PRH1 locus. The As protein could not be discriminated from the parotid isoelectric focusing (PIf) protein by isoelectric focusing gel electrophoresis due to similar migration of the two proteins. In order to determine salivary PRH1 phenotypes it is necessary to use SDS or basic gel electrophoresis in addition to the isoelectric focusing gel electrophoresis. The As protein was not found in Caucasians. The allele frequencies of the PRH1 locus in Japanese were PRH11 (double-band protein) = 0.035, PRHl2 (acidic protein) = 0.193, PRH14 (PIf) = 0.751, and PRH15 (As) = 0.021.