Mutans streptococci exhibit an absolute requirement for manganese for expression of the dextran-dependent aggregation (DDAG) phenotype. Glucan-binding protein C (GbpC) has been demonstrated to be involved in the DDAG phenotype and adhesion of S. mutans. Objectives: It was of interest to analyze the relationship between the concentration of Mn2+ in growth medium and the expression levels of GbpC in both biofilm and planktonic cells of S. mutans. Methods: Biofilm and planktonic cells of wild-type S. mutans and gbpCֿ mutant were grown in Mn-depleted medium +/- 100 mM MnSO4. GbpC was purified from digested cell walls and then analysed by enhanced chemiluminescence immunoblotting with anti-GbpC antiserum and glycoblotting with FITC-dextran. Dextranase-treated cell suspensions were mixed with dextran T2000. Then, the aggregation rate was monitored as a loss of turbidity. Results: The anti-GbpC antiserum reacted with the 59 kDa band and 42-39 kDa doublet in cell wall-extracts of the wild-type strain. These proteins were not detected in cell wall-extracts of gbpCֿ mutant, nor were they detected in the culture supernatants of either gbpCֿ mutant or the wild-type strain. GbpC was expressed from highest to lowest order in Mn-supplemented planktonic cells, Mn-depleted biofilm cells, Mn-supplemented biofilm cells, and Mn-depleted planktonic cells. Glycoblotting confirmed that anti-GbpC positive bands were capable of binding dextrans. The relative levels of DDAG phenotype expressed in cells grown in the four conditions corresponded to the intensities of anti-GbpC positive bands on the immunoblot. Conclusion: Manganese upregurates the expression of GbpC of planktonic cells of S. mutans. This study was supported by the Genecor Inc.