The phenotypic expression of a dominantly inherited human salivary acidic protein (Pa) has been described in acid-urea starch and in Tris-borate acrylamide gel systems. Estimates of the Pa+ allelic frequencies in American Caucasians, American blacks, and Orientals are .21, .14, and .42, respectively. The genetic and biochemical similarities to another series of proline-rich salivary proteins, Pr, and to a pair of similarly staining salivary proteins, Db (double band), are evaluated. It is concluded that either one locus or two (or three) tightly linked loci are viable explanations for this polymorphic system(s). It is suggested that the three factors, Pa, Pr, and Db, be treated as separate loci to allow clarification of their genetic relationships.