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Flow-cytometric T cell crossmatch (FTXM) uses live donor lymphocytes to detect anti-donor-human leukocyte antigen (HLA) IgG antibodies. Therefore, the results can be affected by individual cell surface molecular expression. Herein we report three cases with FTXM false-positive living-related kidney transplants whose positivity was influenced by the expression of incompatible ABO blood-type antigens.The kidney recipients were three men with blood type O who received from their wives with blood type B. Their anti-B IgG titers were 1:32, 1:128, and 1:256. Lymphocyte cytotoxic assays were negative in all cases. Median of mean channel shift (MCS) in FTXM was 78.7% (range, 52.8%–85.7%). There were no anti-HLA (either donor-specific or not) class I IgG antibodies in all 3 cases detected by a solid-phase bead assay. We hypothesized that non-HLA antigens expressed on T lymphocytes affect false-positive FTXM results. To examine this, we incubated the patients’ sera with donor-type red blood cells (RBCs) to adsorb anti-ABO antibodies prior to FTXM.When the incubated sera were tested in FTXM with donor T lymphocytes, the results were negative in all cases with median MCS 3.75% (0.0%–7.5%). To clarify the expression of ABO antigens on the donor T lymphocytes, we incubated donor T lymphocytes with recipients’ sera and then incubated the cells with acid solution to detach the bound antibodies from the T lymphocytes. The eluates showed agglutination in indirect Coombs test with blood type B RBCs but no agglutination with O blood type RBCs. This indicates that the donor T lymphocytes expressed blood type B antigen and anti-blood type IgG antibodies reacted with the B antigen. These results show that anti-blood type IgG antibodies can bind to ABO-blood type antigens on the surface of T lymphocytes, resulting in positive FTXM. The false-positive FTXM can be excluded by short-term incubation of recipients’ sera with donor-type RBCs.In ABO-incompatible kidney transplantation, anti-blood type IgG antibodies can be detected as anti-donor IgG antibodies in FTXM.