The bursa of Fabricius is a primary lymphoid organ in birds responsible for B lymphocyte maturation. Sex steroid hormones influence regression of the bursa of Fabricius. Bisphenol A (BPA) is an estrogenic chemical that is widely used in the manufacture of plastics and epoxy resins. This experiment was conducted to determine the effects of BPA on the involution of bursa of Fabricius in the adult hens. The synthetic estrogen diethylstilbestrol (DES) was used as a positive control. The test compounds were injected into the yolk of embryonated eggs. Immunohistochemical procedure performed to determine estrogen receptor alpha (ERα) expression in bursa of Fabricius. In high dose BPA (134 μg/g egg) and DES (0.2 μg/g egg) groups, the bursa somatic index, the number and diameter of the follicles in bursa of Fabricius were statistically decreased compared to control and low dose BPA (67 μg/g egg) and DES (0.02 μg/g egg) groups (P<0.05). There were no specific immune reactions for ERα in bursa of Fabricius in all groups. These results suggest that embryonic exposure to BPA and DES induced the regression of the bursa of Fabricius in hens.