Rats and mice infected with Trypanosoma lewisi and Trypanosoma musculi, respectively, showed hyperreactivity to lipopolysaccharide (LPS) from gram-negative bacteria. Fatal shock could be precipitated with a dose of LPS 100 to 1,000 times less in infected compared with noninfected animals. In trypanosome-infected rats and mice, extensive liver damage was evident after LPS challenge. These animals showed a pronounced hypoglycemia, marked elevation of blood aspartate transaminase level, and diffuse severe degeneration and total depletion of glycogen in hepatocytes. Only minor changes were observed in noninfected animals given the same dose of LPS. No mononuclear phagocytic cell infiltration was observed in the liver of infected animals. The most striking change was the great increase in size and the probable increase in phagocytic activity and number of sinusoidal Kupffer cells. We suggest that elevated Kupffer cell activity in trypanosome-infected animals may play a role in LPS-induced hepatotoxicity.