A series of prospective ocular permeation enhancers, benzalkonium chloride (BAC), cetylpyridinium chloride (CPC), ethylenediaminetetraacetic acid (EDTA), polyoxyethylene (20) stearyl ether (PSE) and polyethoxylated castor oil (PCO) were tested for cytotoxicity on cultures of rabbit (RCE) and human (HCE) corneal epithelial cells. The cells were treated for 5,15 and 60 min with different concentrations of the test substances, in serum-free medium and in medium containing 15% foetal bovine serum (FBS). The cytotoxicity was evaluated by WST-1 test. The EC(50) values for HCE, after 15 min exposure and in the presence of FBS, indicate the following order of cytotoxicity: PSE> or =BAC>CPC>EDTA>PCO. After 1 h exposure the order of decreasing cytotoxicity was PSE> or =BAC>CPC>PCO>EDTA. In all cases the presence of FBS appeared to exert a protective effect against the cytotoxic effect.