The human mouth is an important route of viral transmission and evidence exists that human saliva can neutralize some viruses, e.g. herpes simplex type 1 (HSV-1) and human immunodeficiency virus (HIV) in vitro. However, little is known of the actual antiviral agents in saliva. We have analyzed how hypothiocyanite (HOSCN/ - OSCN) ions, present in human saliva and generated by salivary peroxidase systems, affect the viability of three different types of viruses: HSV-1 (capable of inducing oral lesions), respiratory syncytial virus (RSV, respiratory infections), and echovirus 11 (EV 11, enteric diseases). Viral suspensions were pretreated (30 min) with HOSCN/ - OSCN concentrations up to 180 μM both at pH 6.0 and 7.1 and inoculated into human gingival fibroblasts. The cultures were incubated at 37°C for 18-48 h, fixed and the infected cells were counted after immunoperoxidase staining. HSV-1 was most sensitive to HOSCN/ - OSCN with an IC 5 0 of 8.5 μM at pH 6.0 and an IC 5 0 of 20 μM at pH 7.1, respectively. RSV was inhibited by HOSCN/ - OSCN only at pH 6.0 with an IC 5 0 of 8.0 μM. EV 11 was also resistant at neutral pH, but sensitive at pH 6.0 with an IC 5 0 of 68 μM. In contrast to HSV-1 and RSV, the inhibition of EV 11 was not dependent on the concentration of HOSCN/ - OSCN. The inhibition was in all cases stronger at pH 6.0 than at neutral pH. Our results suggest that hypothiocyanite, a normal component of human whole saliva, in physiological concentrations effectively inhibits HSV-1 and RSV at acidic pH, whereas EV 11 is more resistant in vitro.