Ozone has been proposed as an alternative antiseptic agent in dentistry based on reports of its antimicrobial effects in both gaseous and aqueous forms. This study investigated whether gaseous ozone (4 x 10(6) microg m(-3)) and aqueous ozone (1.25-20 microg ml(-1)) exert any cytotoxic effects on human oral epithelial (BHY) cells and gingival fibroblast (HGF-1) cells compared with established antiseptics [chlorhexidine digluconate (CHX) 2%, 0.2%; sodium hypochlorite (NaOCl) 5.25%, 2.25%; hydrogen peroxide (H(2)O(2)) 3%], over a time of 1 min, and compared with the antibiotic, metronidazole, over 24 h. Cell counts, metabolic activity, Sp-1 binding, actin levels, and apoptosis were evaluated. Ozone gas was found to have toxic effects on both cell types. Essentially no cytotoxic signs were observed for aqueous ozone. CHX (2%, 0.2%) was highly toxic to BHY cells, and slightly (2%) and non-toxic (0.2%) to HGF-1 cells. NaOCl and H(2)O(2) resulted in markedly reduced cell viability (BHY, HGF-1), whereas metronidazole displayed mild toxicity only to BHY cells. Taken together, aqueous ozone revealed the highest level of biocompatibility of the tested antiseptics.