Tumor-specific cytotoxic T cells (CTLs) can play an important role against cancer as illustrated by the observation that adoptive transfer of tumor-specific CTLs can mediate potent anti-tumor effects. Although such CTLs can be detected at the tumor site, relatively little is known about the mechanisms by which they enter the tumor. In this study, the role of major histocompatibility complex (MHC) class I molecules on vascular endothelium in the tumor in entry of, and tumor eradication by, tumor-specific CTL was investigated. Two H-2Db-restricted CTL clones recognizing peptide VNlRNCCYl on human adenovirus type 5 early region I -(Ad5E I)-induced tumors were used to test whether CTLs were able to cross the vascular endothelium lacking the restricting MHC molecule. One CTL clone recognizes peptide VNlRNCCYl in the context of both H-2Db and H-2Dbmi4 molecules. The other CTL clone recognizes this peptide only in the context of H-2Db. Adoptive transfer of these CTLs leads to eradication of Ad5E I -induced, H-2Db-expressing tumors in B6 (H-2Db+) and Bm 14 (H-2Db-) nude mice. Our data show that presentation of tumor-derived peptides by MHC molecules on endothelial cells of blood vessels in a tumor do not play a major role in eradication of tumors by adoptively transferred CTL in combination with interleukin-2. Moreover, our data show that successful adoptive CTL immunotherapy is possible across allogeneic barriers, without inducing severe side effects, provided the tumor expresses the MHC class I-peptide complex recognized by the CTLs.