A panel of monoclonal antibodies was produced to study the mesenchymal stromal elements of the bursa of Fabricius in guinea hen. The intracellular antigens recognized by GIIF3 and NIC2 monoclonal antibodies are of 50 and 30 kD, respectively. The cells identified by these antibodies emerge in the mesenchyme around day 12 of incubation, and immigrate into the surface epithelium of the bursal folds, which precedes the follicle formation. The GIIF3 cells move up to the luminal surface of the follicle and differentiate to follicle-associated epithelial cells. The NIC2 cells remain in the medulla, produce and secrete large amount NIC2 positive substance, when the bursal function starts up. The presence of double positive (GIIF3 and NIC2) cells in the medulla around hatching, seems to indicate, that the two cells might have a common origin. The NIC2 positive product of the bursal secretory dendritic cells contributes to the microenvironment, and possibly necessary for the B cell clonal expansion and establisment of the immune repertoire in the guinea hen.