Major histocompatibility complex class I-deficient islets from beta-2 microglobulin-deficient mice have been shown to have prolonged in vivo islet allograft survival. Additionally in vitro studies using the mixed lymphocyte islet coculture system have demonstrated a reduction in cytotoxic T lymphocyte activity against alloislets that have been pretreated with an antibody directed against MHC class I antigen. The clinical applicability of these findings are examined in this study, which evaluates the ability of MHC class I blocking antibody to prevent the in vivo destruction of alloislets. Recipients of allogenic islet transplants treated with phosphate-buffered saline or control F(ab')2 fragments rejected the transplanted alloislets in median times of 11.5 days and 11 days, respectively. Recipient mice treated with a monoclonal antibody or F(ab')2 fragments specific to the donor MHC class I antigen had significant prolongation in allograft survival (median allograft survival for both groups was 21 days) when compared with mice treated with PBS or control F(ab')2 fragment. These results demonstrate that treating recipients of alloislets with donor-specific MHC class I monoclonal antibody or the respective F(ab')2 fragments prolongs islet allograft survival. This confirms the importance of MHC class I antigen in the rejection of pancreatic islet allografts and suggests that blocking different domains on the MHC class I molecule could be used therapeutically in the protection of allografts from the immune system.