Cell-surface antigens of Clostridium difficile and LPS from Escherichia coli were investigated for modulating effects on the activity of C. difficile toxin A on Vero and Caco2 cells. The antigens of C. difficile tested comprised: (i) an EDTA extract, which contained several major and minor cell-surface proteins and the membrane-associated lipocarbohydrate (LC); (ii) a guanidine hydrochloride extract, which mainly contained the surface-layer proteins; (iii) an aqueous phenol-extracted, protein-free LC. On their own, none of the antigens had a detrimental effect on the cells, with the EDTA extract and LC having a marginally protective effect. When these antigens were added to suboptimal levels of toxin A, there was significant enhancement of its cytotoxicity by the EDTA and LC preparations on both cell types. LPS showed some enhancement of the effect of toxin on Vero cells at the lowest levels of toxin investigated. It was concluded that this effect seen in vitro may have a role to play in the colon during infection with C. difficile.