Since aortic allografts have been cryopreserved, aortic allograft implantation has generally been considered to show stable and sufficient outcome in clinical cardiovascular surgery. Compared with gluteraldehyde-treated xenografts, aortic allografts have several benefits; for instance, durability, thrombogenicity, and resistance to infection. Furthermore, patients who have undergone aortic allograft implantation do not require anticoagulant therapy.When allogeneic tissues or organs are implanted into recipients, it is necessary to consider antigenicity so as to prevent immune response and rejection. Some investigators have suggested that allogeneic tissues have less antigenicity when they are cryopreserved. Cryopreserved tissue cannot easily induce immune response in recipients, and its function is preserved. On the other hand, Yacoub et al performed implantation of fresh aortic allografts into the aortic roots of patients, and showed results that were almost the same as those of implanted cryopreserved allografts. Thus, it is controversial as to whether or not cryopreservation suppresses antigenicity of allogeneic tissue.In the present study, we have elucidated both class I and class II antigenicities of cryopreserved tail skin by using a class I antigen alone and disparate combination of B6.C-H-2 b m 1 (bm1; K b m 1 , IA b , D b ) and C57BL/6 Cr Slc (B6; H-2 b ), and class II antigen alone and disparate B6.C-H-2 b m 1 2 (bm12; K b , IA b m 1 2 , D b ) and B6. Because skin grafting has been used as an indicator of in vivo immunity, we believed it was important to initially obtain definite results by using class I or class II antigen-disparate congenic mice and using skin allograft as tissue in mice. In the combination of bm1 and bm12 mice → B6 mice, CD8 + T cells and CD4 + T cells have been shown to be effectors in in vitro assay and skin allograft rejection, respectively. Results indicate that both class I and class II antigenicities show no difference between cryopserved and fresh skin allografts, giving evidence that cryopreservation has no effect on expression of class I or class II antigens.