Low-level chemiluminescence (CL) appeared on addition of hydrogen peroxide to human saliva and then decayed slowly. Azide (10 microM) inhibited CL by about 50% and deuterium oxide (99.75%) enhanced it about twofold. 1,4-Diazabicyclo[2,2,2]octane (50 mM) and tryptophan (10 mM) were also enhancing, which suggests that singlet oxygen participates in this CL. The optimal pH for CL was around 8.5. Molecular oxygen was produced on addition of hydrogen peroxide to human saliva with a time course similar to that of CL; the optimal pH for oxygen evolution was around 8.0. The levels of SCN- and OSCN- at first decreased and increased, respectively, on addition of hydrogen peroxide and then remained constant as long as the induced CL could be detected. Dithiothreitol (1 mM) and mercaptoethanol (1 mM) completely suppressed CL. Induced CL was observed in saliva dialysed against 10 mM sodium phosphate (pH 7.5). Its intensity was increased by NaSCN, reaching a maximum around 0.1 mM NaSCN in the presence of 0.2 mM hydrogen peroxide. These results suggest that part of the molecular oxygen evolved on addition of hydrogen peroxide to human saliva is in a singlet state and that molecular oxygen is evolved by oxidation of hydrogen peroxide, which may be catalysed by OSCN- bound to salivary peroxidase.