The hypothiocyanite ion (OSCN-) is the principal oxidation product of the salivary peroxidase-thiocyanate (SCN-)-hydrogen peroxide antimicrobial system. Supplementation of human saliva in vitro and in vivo with low amounts (less than 1.0 mM) of hydrogen peroxide increase the concentration of salivary OSCN- (in vivo up to 0.3 mM). Elevated concentrations of OSCN- are strongly antimicrobial and may therefore be protective against dental caries. However, as OSCN- is a highly-reactive oxidizing agent, its possible toxic effect on human cells was studied using gingival fibroblasts as target cells. Concentrations of OSCN- (up to 300 microM) had no effect on [3H]-thymidine incorporation into the cells. However, fibroblasts were sensitive to peroxide so that 100 microM of H2O2 caused over 80 per cent reduction in [3H]-thymidine incorporation. The toxicity of H2O2 could be entirely prevented by adding lactoperoxidase and SCN- to the cell culture before the addition of peroxide. Thus, conversion of toxic H2O2 to non-toxic OSCN- in fibroblast culture by lactoperoxidase and SCN- suggests a dual role for the salivary peroxidase system: protection of human cells from H2O2 toxicity and antimicrobial action against oral pathogens. Furthermore, the elevated concentrations of OSCN- which produce inhibition of bacterial metabolism did not damage human cells.