The cytotoxicity against two mesenchymal cell lines, L-929 and WI-38, and two epithelial cell lines, KB and HeLa S-3, of lipopolysaccharide (LPS) purified from strains of Bacteroides gingivalis, Fusobacterium nucleatum, and Veillonella parvula isolated from infected root canals was investigated. The inhibition of cell growth by these LPS's was considerable for mesenchymal cell lines, but mild for epithelial cell lines. The cytotoxic effect of F. nucleatum LPS was the greatest and that of B. gingivalis LPS was the least.