Cultured human and rat corneal epithelial cells with51cr incorporated were used as a model to test the cytolytic action of four common preservatives. Benzalkonium chloride, chlorohexidine and thimerosol were all found to lyse>40% cells when incubated for fifteen minutes at concentrations in clinical use in topical ophthalmic medications. Chloro-butanol is the only preservative tested which has a low level of cytotoxicity (10%) and which, under these conditions, can be considered a safe preservative using cytolytic activity as the means of criteria.