The aim of this study was to investigate the effects of in ovo administrated testosterone propionate (TP) on the embryonic development of bursa of Fabricius and also to determine the histological changes occuring at post-hathcing period. In the study, totally 350 fertilized eggs of a hybrid breed, B55 were used. The eggs were divided into two groups. First-group eggs were served as controls, in the second group-eggs, which were treated with testosterone propionate, constituted experimental. Light microscopic observations have showed that in ovo hormonal bursectomy adversely affected lymphoid follicle development via blocking the epithelial bud formation. Nevertheless, TP-treatment did not completely prevented follicle formation in the organ; atrophic and degenerated follicfes with cysts were also seen, although they were in small numbers. In TP-treated group, the organ was smaller than those of conlrols and most of the lymphoid follicles have not completed their development. At the first week of post-hatching period lymphoid infiltration areas and cysts were observed in the lamina propria of the organ. At the 8th week of post-hatching period, a rudiment of the organ which was mainly constituted of the connective tissue containing diffuse lymphoid in-fillration areas was observed.