An oral administration of partially purified LPSw, a lipopolysaccharide (LPS) from wheat flour, at a concentration of 20 ng/ml in drinking water beginning 1d after infection significantly decreased mouse mortality and prevented animal weight loss in acute infection with Toxoplasma gondii. Whereas 71% (5/7) of mice in a control group that did not receive LPSw died of toxoplasmosis, only 14% (1/7) of mice treated with LPSw died (p less than 0.05). The administration of LPS purified from Bordetella pertussis also significantly decreased the mortality of infected mice. LPS from Escherichia coli and synthetic lipid A (LA-15-PP(506)), however, did not show a significant decrease in mortality.