Leukoreduction prevents transfusion-induced rejection of HY⁺ BMT. Transfusion of whole blood but not leukoreduced blood from HY⁺ (male) donors into female B6.Thy1.1 causes rejection of HY⁺ BMT. Injection with male whole blood induced rejection in 93% of the recipients compared with untransfused mice or mice transfused with syngeneic blood (P = .001, 2-tailed Fisher exact test). (A) Transfusions with male LR blood did not significantly affect the number of mice displaying BMT engraftment. (B) Chimerism in engrafted animals was variable with the observation of microchimerism in some animals. Data shown are the compounded results of 2 separate experiments.