Novel biocides, which avoid the induction of cross-resistance to antibiotics, are an urgent societal requirement. Here, we compared the cytotoxic and bactericidal effects of a new antimicrobial agent, the iodo-thiocyanate complex (ITC), with those of the common antiseptics, hydrogen peroxide (H2O2), povidone iodine (PVP-I) and Lugol's iodine (Lugol). The antimicrobials were co-incubated for 10 min with HeLa and Escherichia coli. cells in the presence and absence of organic matter (Dulbecco's modified Eagle's medium, supplemented with 10% fetal bovine serum). The cytotoxic concentrations of ITC were equivalent to its bactericidal concentrations (7.8 mu g ml(-1)). By contrast, cytotoxic effects of H2O2, PVP-I and Lugol were apparent at concentrations lower than their bactericidal concentrations (250, 250 and 125 mu g ml(-1), respectively). The cellular effects of ITC were not quenched by organic matter, unlike the other antiseptics. ITC, PVP-I and Lugol had hemolytic effect on horse erythrocytes at high concentrations, while H2O2 showed no hemolysis. ITC, at 30 or 300 mu g ml(-1), did not cause DNA breakage in HeLa cells as assessed by an in vitro comet assay in the absence of S9 metabolic activation, whereas H2O2 caused extensive single-strand DNA breaks. The pronounced antimicrobial potency of ITC and its favorable cytotoxicity profile suggests that ITC should be considered for antiseptic applications.