Mice infected subcutaneously with 5 X 10(3) viable cells of Escherichia coli ATCC 25922 incorporated in liquified agar developed a systemic infection. Increasing bacterial numbers could be recovered from the liver at several days following infection. Ultimately, the animals died 6 days after infection. Treatment of mice with 1 mg killed Propionibacterium granulosum KP-45 lead to an increased spleen weight at day 7 after intraperitoneal injection. Hence, the animals were highly susceptible to the lethal action of endotoxin. They were, however, markedly protected against infection with E. coli, since definitely lower bacterial counts were found in the liver of pretreated mice in comparison to controls.