Sex steroid sensitivity of the bursa of Fabricius (BF) was studied from the early embryonic time until its regression. Expression of progesterone receptor (PR) served as a dual marker: first, as a marker for progesterone sensitivity and second, as a marker for estrogen action, since it is an estrogen-induced protein. The progesterone binding molecule in the bursa was characterized by different chromatography methods and by steroid binding studies. We showed that it fulfils the criteria of a progesterone receptor by binding, structural and immunological properties. With immunohistochemistry and with the combined techniques of immunohistochemistry and autoradiography we demonstrated two cell types which express the PR: smooth muscle cells surrounding the BF and stromal cells located under the bursal epithelium and between the lymphoid follicles. The epithelium and the cells inside the lymphoid follicles were negative. Using immunoelectron microscopy the PR-expressing stromal cells were shown to be fibroblasts. The cloacal mesenchyme, from which the BF develops, was shown to be sensitive to exogenous estrogen very early during the embryonic time. The mesenchyme around and inside the developing BF reached estrogen sensitivity a few days later. The estrogen-sensitive mesenchymal cells were first seen surrounding the bursal primordium and later in the center of the plicae. During a natural sexual maturation without exogenous estradiol an expression of the PR was detected much later, at the age of 10-12 weeks after hatching. This expression correlates with the onset of the bursal regression and with the increase of the sex steroid levels in the blood. In the oviduct stroma PR was undetectable before the onset of sexual maturation. In the oviduct stroma PR becomes detectable a few weeks earlier than in the bursa.