Triclosan and sodium lauryl sulfate (SLS) are antimicrobial agents used, both singularly and in combination, in dentifrices and mouth-rinses. Studies by Waaler et al. (Scand. J. Dent. Res. 101 (1993) 192-195) with human volunteers showed that the adverse side-effects induced by SLS in mouth-rinses, i.e. desquamation of oral epithelium and a burning sensation, were lessened by the addition of triclosan. However, Baert et al. (Int. J. Exp. Pathol. 77 (1996) 73-78) showed that triclosan did not protect the hamster cheek pouch mucosa from irritation caused by SLS. The studies presented herein further evaluated, using a cell culture system, the triclosan-SLS interaction. The in vitro cytotoxicities of triclosan and SLS, alone and in combination, were determined with human gingival S-G epithelial cells and GF fibroblasts. The 24-h midpoint (NR50) cytotoxicity values towards the S-G cells were 0.052 mM triclosan and 0.0075% SLS and for the GF fibroblasts the respective values were 0.095 mM triclosan and 0.0127% SLS. Both agents at their NR50 values induced vacuolization. Coexposures of triclosan and SLS were additive in their cytotoxicities towards the S-G epithelial cells and GF fibroblasts. Pretreatment with triclosan potentiated the toxicity of a subsequent exposure of SLS to the S-G cells; a similar pretreatment of the GF fibroblasts with triclosan had no effect on a subsequent challenge with SLS.