The esterase activity in the bursa of Fabricius in 10-day-old chicks is examined by means of serial sections obtained by cutting along the major axis of the lymphoid follicles. The positivity for this enzymatic reaction is not only confined to the FAE (follicle associated epithelial) cells of the epithelial frame-work and to isolated cells belonging to the medullary part of the follicle, but it extends in a club-shaped morphological continuity from the FAE cells towards the centre of the follicle. A similar image with a high affinity for acid stainings was also found by means of the H and E technique in suitably orientated sections. Morphological continuity between the FAE cells and the centre-medullary part leads us to suppose the FAE cells and the cells of the centre-medullary part may have a common origin. In the present work, we consider the possibility of FAE cells being of mesenchymatic origin because, besides the fact that they are in continuity with the centre-medullary cells, they appear after CFU (Colony Forming Units) migration, they have no basal lamina, they have certain features which are present in mesenchymatic cells belonging to the histiocytic line, like high esterase activity and micropinocytotic capability and, finally, they are sensitive to carrageenan and silica, which is a peculiarity of macrophages.