Abstract: By applying colloidal carbon on anal lips of the chicks,it is found that the carbon particles are endocytized by the follicle-associated epithelial cells of the buras of Fabricius.After 24 hours,the endocytized carbon particles present mainly in the stellate cells and the intercellular spaces in the medulla of the lymphoid follicles.The follicle-associated epithelium (FAE) treated with testosterone propionate (TP) in different developmental stages is found to be damaged.The ultramicrostructure of these cells is changed and endocytosis is apparently inhibited.When TP is treated,especially in higher doses,endocytosis is completely suppressed.In addition,the morphological changes of the FAE treated with TP are observed to be similar tn natural involution of the normal bursa of Fabricius.Thus,the present experimental results give further evidence that natural involution of the bursa of Fabricius in chickens is related to the increasing of the male hormonral concentration in the organism.