A comparative study on the endotoxic effects of lipopolysaccharide (LPS) from Veillonella parvula ATCC 10790 and from Bacteroides intermedius BMH was performed using an in vivo approach in the C57BL/6 mouse. Phenol-water extracted LPS of such anaerobes was purified by ultracentrifugation and DNase/RNase digestion, and characterized by a metachromatic assay for endotoxins and by electrophoresis on SDS-polyacrylamide gel and silver staining. Mouse LD50 for V. parvula LPS was 1.479 mg and for B. intermedius greater than 3.160 mg. Sublethal amounts of the LPS from anaerobes as well as from facultative aerobes decreased daily water intake and body weight in the mouse. Endotoxin from Salmonella typhimurium SL1102, Escherichia coli 0128:B12 and V. parvula had a strong effect on water intake and body weight, whereas Bacteroides intermedius LPS activity was very weak. The results of the present report suggest that V. parvula LPS has a toxic in vivo activity on mouse, which is comparable to LPS from classic enteric organisms and stronger than B. intermedius LPS.