Destruction of target cells by cytotoxic T lymphocytes requires the presence of HLA (human lymphocyte antigen) class I antigens on the target cells for adhesion as well as for triggering of the antigen-specific T cell receptor. Rejection of xenogeneic human pancreatic islets and liver was circumvented by masking, before transplantation, donor antigens with F(ab')2 antibody fragments to HLA class I or tissue-specific epitopes. This strategy eliminated the need for recipient immunosuppression and allowed islet xenograft survival beyond 200 days, as demonstrated functionally by C peptide secretion as well as by histology. These in vivo observations are consistent with the importance of donor HLA class I in eliciting graft rejection and have potential applicability to the successful transplantation of other HLA class I-bearing donor tissues.