The initial step in the lactoperoxidase (LPO) catalysed oxidation of thiocyanate (SCN––) by hydrogen peroxide (H2O2) leads to the formation of hypothiocyanite (OSCN––), the ion of hypothiocyanous acid. In dilute non-acid solutions, the compound is much more stable than previous investigations have suggested. OSCN–– was prepared non-enzymatically, by the hydrolysis of thiocyanogen (SCN)2 in strongly alkaline solution. These solutions were inhibitory towards bacteria, in the same way as the LPO preparations. OSCN–– oxidises thiol groups, and during this reaction SCN–– is regenerated. Since the inhibitory effect of the LPO system has been shown to be due the oxidation of enzymes containing essential thiol groups, it is postulated that OSCN–– is identical to the inhibitor. The over-all mechanism can be explained by an interference in the NADH-NADPH balance of the cell. In the presence of LPO the oxidation of NADH, giving H2O2, results via OSCN–– in the oxidation of thiols. For their reduction again, however, NADPH is required.