Abstract: Daidzein is a phytoestrogen isoflavone found in soybeans and other legumes, and has been implicated as an immune enhancer. Objectives of this study were to determine the effects of daidzein supplementation to weaned pigs on pig growth and response to a lipopolysaccharide (LPS) challenge. Forty crossbred barrows were removed from their sows and allowed a period of 5 d to acclimate to new housing and dry feed. After that time, pigs were weighed, individually penned, and assigned to either daidzein (D; n=20) or control (C; n=20) treatment. The D pigs received 50 mg d-1 daidzein (LC Laboratories, Woburn, MA) hand-fed in dough balls, and C pigs received dough balls without D. After supplementation with D for 14 d, all pigs were weighed, non-surgically cannulated in the jugular vein, and assigned to rectal temperature (RT) measurement (n=10 D, 10 C) or blood collection groups (n=10 D, 10 C). The following day, blood samples and rectal temperatures were collected at 30 min intervals from -1 to 4 hr post-LPS. At time 0, all pigs received a 50 g kg-1 dose of LPS through the jugular cannulae. Serum was collected from all blood samples and stored at -80 C until assayed for cortisol concentrations by radioimmunoassay. Data for BW, ADG, serum concentrations of cortisol, and rectal temperature were analyzed using ANOVA in StatView. There was no difference (P > 0.72) in initial BW, with average BW of 8.36 ? 1.13 and 8.24 ? 0.93 kg for D and C pigs, respectively. There was no difference (P > 0.50) in final BW, with average BW of 15.30 ? 2.30 and 15.73 ? 1.65 kg for D and C pigs, respectively. Accordingly, there was no difference (P = 0.18) in ADG (0.46 ? 0.10 kg d-1 D; 0.50 ? 0.07 kg d-1 C). There was no treatment by time interaction (P > 0.66) for serum concentrations of cortisol; however, there was an effect of time (P < 0.0001), with cortisol concentrations increasing over time. Similar to cortisol, RT increased (P < 0.0001) over time in both D and C pigs. There was a trend (P < 0.12) for decreased RT in D as compared to C pigs, with RT decreasing in D pigs by 4 hrs post-LPS as compared to C pigs that demonstrated no decrease in RT by 4 hrs post-LPS. This study suggests that daidzein might provide some beneficial protection against an immune challenge.