Twenty five specimens of branchial cyst from the same number of patients have been examined. On staining with haematoxylin and eosin a consistent finding was that the mural lymphoid follicles were always aligned with their mantle zones towards the luminal epithelium. With conventional staining lymphatic sinuses were noted in 17 of the specimens, but with immunohistochemical staining these structures were apparent in 23 cysts. Their frequent occurrence in branchial cysts supports the theory that these lesions are derived from epithelial inclusions in lymph nodes. Immunohistochemical techniques for a range of other markers, using polyclonal and monoclonal antisera, showed a distribution of lymphoid and non-lymphoid tissue elements, as seen in lymph nodes and, for example, palatine tonsils. The lining luminal epithelium also shared many features in common with the crypt epithelium of tonsils.