Considering the usefulness of the immunopotentiator from Pantoea agglomerans 1 (IP-PA1), which is a purified lipopolysaccharide (LPS) derived from symbiotic gram-negative bacteria of food crops, in controlling immunosuppression in poultry husbandry, in this study, we examined its immune-recovery effects in dexamethasone-treated stressed chicken models. Three-week-old chickens daily administered 10 microg/kg of dexamethasone for 35 days to induce stress showed more whole body weight loss; relative thymic, bursal, and splenic weight losses; and decrease in the number of peripheral blood lymphocytes, as compared with the control chickens on day 35; the IP-PA1-pretreated, dexamethasone-treated chickens showed reduced weight losses. Five- to eight-week-old chickens administered 5 mg/kg of dexamethasone showed excessive apoptosis of thymic and bursal lymphocytes 24 hr after a single dexamethasone treatment; apoptosis was inhibited in the IP-PA1-pretreated, dexamethasone-treated chickens. Chickens daily administered 10 microg/kg of dexamethasone for 35 days and injected with commercial Salmonella Enteritidis (SE) vaccine or sheep red blood cells (SRBC) on days 7 and 21 showed about 8- or 2-fold lower antibody production in response to SE or SRBC, respectively, as compared with the control chickens on day 35; the antibody production in response to SE or SRBC was increased in the IP-PA1-pretreated, dexamethasone-treated chickens. These results indicate that IP-PA1 exerts inhibitory effects on dexamethasone-induced immunosuppression and that it may be useful in controlling immunosuppression in poultry husbandry.