By exposing primary cultures of human corneal epithelial cells to a single dose of either benzalkonium chloride or chlorobutanol, evaluated the cytotoxicity of these two preservatives which are commonly used in artificial tear solutions and other topical ophthalmic medications. Control and experimental cultures were analyzed by continuous time-lapse videomicrographic recordings as well as by sequential phase-contrast microscopy. Both benzalkonium chloride at a concentration of 0.01% and chlorobutanol at 0.5% caused immediate cessation of normal cytokinesis and mitotic activity, and epithelial cells degenerated within two hours and eight hours, respectively.