AbstractThe cavernous body of the ammocoetes of the anadromous sea lampreyPetromyzon marinuswas ultrastructurally analyzed with special emphasis on its lymphoid components. It is a vascular structure differentiated from the afferent artery of each gill filament and contains a specialized phagocytic endothelial cell, named the cavernous body (CB) cell. In the vascular spaces of the cavernous body there occur numerous lymphoid cells, including lymphocytes, lymphoblasts, mature and developing plasma cells and large macrophages, which make contact with the CB cells. The role of these cells is discussed as to the suggested functions of the cavernous body in the defence mechanisms of lampreys.