Immunohistochemical characterization of microvascular endothelial cells in primary B cell lymphoma of the gastrointestinal tract. Microvascular endothelial cells (EnC) in primary B cell lymphoma of the gastrointestinal tract were immunohistochemically studied. Based on the morphological structure, the microvasculatures were divided into high endothelial cell vessels (HEV) and flat endothelial cell vessels (FEV). There were distinct phenotypic differences between HEV and FEV in lymphoma tissues. HEV were characterized as OKM1- OKM5-, accompanied by the cluster of non-neoplastic T lymphocytes, and FEV were OKM1- OKM5+ not accompanied by T lymphocyte infiltration. Factor VIII-related antigen was clearly identified in both EnC, and major histocompatibility complex (MHC) class II antigens and interleukin 1 were absent or only faintly visible on EnC in lymphoma tissues, whereas they were expressed on EnC in non-lymphoma tissues. These findings suggest that microvascular EnC in primary B cell lymphoma of the gastrointestinal tract lack a property as antigen-presenting cells, and that HEV are involved in the migration of non-neoplastic T lymphocytes. 