These studies test the hypothesis that vascular endothelia in sponge allografts may develop a function and phenotype resembling the high endothelial venules (HEV) in lymph nodes, thus facilitating the lymphocytic infiltration that is characteristic of allograft rejection. Using limiting dilution analysis to quantitate helper-T-cell accumulation at graft sites, immunohistologic analysis of graft reactivity with the HEV-specific monoclonal antibody MECA 325, and ex vivo lymphocyte-endothelial adhesion assays with graft tissues, we obtained evidence to suggest that HEV-like endothelia may develop at a graft site but that the process whereby lymphocytes accumulate at a graft site is more complex than was initially expected.