The development of an uncontrolled inflammatory response has been implicated in the pathogenesis of adult respiratory distress syndrome and multiple-organ failure. Because zymosan activates complement and induces a systemic inflammatory response, the effect of zymosan on intestinal structure and barrier function was measured in normally nourished (NN) and protein malnourished (PM) mice. Normally nourished and protein malnourished (up to 21 days) mice challenged intraperitoneally with zymosan (0.1 mg/g body weight) were killed 24 hours after zymosan challenge and their organs cultured for translocating bacteria. Zymosan-induced bacterial translocation was limited to the mesenteric lymph nodes of the NN mice, whereas translocating bacteria spread from the gut to the liver, spleen, and blood stream (p less than 0.05) in the PM mice. Zymosan-induced bacterial translocation appeared to be related primarily to the combination of mucosal injury and a disruption of the gut flora ecology in the PM mice and to mucosal injury in the NN mice. The extent of mucosal injury was greater the longer the mice were protein malnourished before zymosan challenge. The effect of zymosan on survival was measured in separate groups of mice. At a dose of 0.1 mg/g body weight, no deaths occurred in NN mice or in 7-day PM mice. However 20% of the 14-day PM mice and 80% of the 21-day PM mice receiving zymosan died. Thus PM predisposes to mucosal damage and the development of potentially lethal gut origin septic state during periods of systemic inflammation.