HeLa cells damaged by exposure to low pH released a diffusible attractant which greatly increased the collision frequency and hence the attachment of chemotactic Salmonella typhimurium. The attractant was tentatively identified as glycine. In contrast, when undamaged HeLa cells were used, no difference was found between a chemotactic parent strain and a non-chemotactic mutant in collision frequency or attachment. It is suggested that factors increasing cell membrane permeability could attract S. typhimurium to host cells in vivo.