No organ equivalent to the bursa of Fabricius in birds, which is responsible for the production of B cells, has been found in mammals thus far. It has been suggested that Peyer's patches and the appendix along with the bone marrow play this role. In this study, the gland of Aselli of shrews was examined, whose function has not yet been clarified. The results of the study of the gland of Aselli in common and water shrews suggest this gland to be the key lymphoid organ analogous to the bursa of Fabricius. However, unlike the bursa of Fabricius, the gland of Aselli seems to fulfil two functions. It not only produces B cells, but also is the site of their terminal differentiation into plasma cells.