Human saliva contains various kinds of proline-rich proteins (PRP) which can be grouped into two types (type 1 and type 2). Although PRPs of both types have a high proline content (usually greater than 30%) and repeating units in their amino acid sequences, they are different in the structure of the repeating units and amino acid compositions. PRPs of type1 have the repeating unit of PPGKPQGPP and are deficient in hydrophobic amino acids, while the repeating unit of PRPs of type 2 is GPGXX`(P)<sub>n</sub> (where X and X` are any amino acid) and hydrophobic amino acids are present. Although PRPs of type 1 have been studied extensively by several investigators, PRPs of type 2 have been studied mainly by our group. In this review I will focus on the chemical, physicochemical, and genetic aspects of type 2 PRPs, and discuss possible developments in this field.