Antigen I/II of Streptococcus mutans is a cell surface protein involved in the adherence of cells to tooth surfaces. In this study, an antigen I/II homologous gene, pah, was identified and sequenced from Streptococcus downei MFe28 using degenerate polymerase chain reaction (PCR) and the gene-walking method. The pah gene encodes a cell-wall-anchoring protein, PAh, containing 1565 amino acids. At the deduced amino acid sequence level, PAh shows a strong similarity to PAg of S. sobrinus (97.6% identity). Southern hybridization analysis indicated that a single copy of the pah gene was preserved in the chromosomal DNA of S. downei. Two pah mutants, SES-1 and SES-2, were constructed and analyzed by Western blotting. Two bands corresponding to 200- and 160-kDa proteins were observed in the parent strain, whereas no band was detected in pah mutant strains. In an adhesion assay of cells, pah mutants failed to adhere to tube surfaces in contrast to the parent strain. Furthermore, saliva-induced aggregation was decreased in pah mutants compared to the parent strain. Together, PAh is associated with the adhesion of cells to abiotic surfaces and whole saliva.