Human lymphoma derived monoclonal antibody (anti "mucosal vessel associated antigen' [MVAA]) binds to high endothelial venules (HEV) in gut-associated lymphoid tissue, but shows only weak reactivity with HEV in peripheral lymphoid tissues. We have used immunohistochemistry and immunoelectron microscopy to define the precise ultrastructural distribution of the molecule, and to determine whether there is any association between this molecule and cellular traffic. We have observed that MVAA is a component of basement membrane which is only expressed by a subset of blood vessels. Although it is restricted to vessels which support lymphocyte traffic within lymphoid tissue, we did not observe any association between the distribution of MVAA and extravasating lymphocytes. MVAA is expressed in the fetal intestine in association with a subset of larger vessels. It is therefore not necessarily induced as a consequence of antigenic challenge. It is likely that MVAA has a structural role related to its restricted microanatomical distribution; possibly the maintenance of integrity of vessel walls which are continuously disrupted by the extravasation of lymphocytes.