A 2-week experiment was conducted to detect antigenic soy proteins in plasma of pigs fed soybean meal (SBM) and fermented soybean meal (FSBM). The suitability of a commercial FSBM to replace SBM in diets for newly weaned pig was also evaluated. A total of 160 pigs were randomly assigned to 5 dietary treatments (n= 8): 400. g/kg soybean meal (FSBM-0), its replacement by protein from fermented soybean meal as 1/3 (FSBM-1), 2/3 (FSBM-2), 3/3 (FSBM-3), and no soy protein in diet (PCON). Diets were fed at weaning, but feed intake (ADFI) and weight gain (ADG) record began after 3 days. Overall, pigs fed PCON had greater (P<0.001) performance and less diarrhea than pigs fed soy proteins. Pigs fed diets with FSBM had less (P<0.001) diarrhea than those fed the FSBM-0 diet. Western blot analysis showed that pig plasma recognized α, α' and β subunits of β-conglycinin and basic subunits of glycinin present in SBM, while acidic subunits of glycinin, and P34 (human immunodominant soybean allergen) showed a weak immunoreactivity. Plasma from pigs fed FSBM exhibited reduced immunoreactivity toward α and α' subunits of β-conglycinin and acidic subunits of glycinin, due to a partial hydrolysis of those proteins during fermentation. In agreement with that observation, analysis of soy proteins in FSBM by sodium-dodecyl sulfate-polyacrylamide gel electrophoresis showed partial digestion of large proteins with antigenic activity. A linear regression analysis of values from enzyme linked immunosorbent assay (ELISA) did not support the notion that anti-soy immunoreactivity in pig plasma changed significantly as the dietary inclusion of FSBM increased. The FSBM used in this study may be used in substitution of SBM protein without detrimental effect on pig performance. Moreover, inclusion of FSBM in diets for newly weaned pigs may reduce diarrhea after weaning. However, optimization of the fermentation conditions in the commercial FSBM is still needed to produce a more hypoallergenic soybean product for young pigs. © 2010 Elsevier B.V.