151 Introduction: Patients can develop donor-reactive antibodies post-transplant, although the pathologic significance of these are controversial. Donor-reactive antibodies often develop in murine cardiac allograft recipients that accept grafts due to short-term treatment with agents such as anti-CD4 mAb or gailium nitrate (GN), but are frequently not investigated. This study characterizes the generation of donor-reactive alloantibodies in mice that have accepted cardiac allografts, and relates these alloantibodies to the incidence of graft acceptance and to the development of transplant vascular sclerosis (TVS). Methods: DBA/2 cardiac allografts were heterotopically implanted into C57BL/6 recipients, which were treated peri-transplant with the anti-CD4 mAb, GK1.5, or with gallium nitrate (GN) for 28 days to permit long-term graft survival. At 60 days post transplant, sera were collected and the allografts harvested from recipients with functioning grafts. Donor-reactive alloantibodies were assessed by flow cytometry. The extent of TVS was determined using trichromic and elastin staining of sections from the midsection of the heart. Results: All of the animals in this study had graft survival>60 days, while untreated recipients rejected their grafts in 7-10 days. Approximately 80% of GN-treated and 50% of GK1.5-treated allograft recipients developed donor-reactive IgG alloantibodies. In animals which developed IgG alloantibodies, titers generally were >1/250, and all IgG isotypes were represented, although alloantibodies of the IgG1 isotype were the most prevalent in GN-treated recipients, with few animals producing high levels of IgG2a or IgG2b. GK1.5-treated recipients did not exhibit any prevalence of isotype production. Interestingly, all of the grafts developed TVS and interstitial fibrosis, regardless of the antibody level or isotype. Conclusions: Our data indicate that avoidance of acute rejection does not imply a lack of allosensitization, since donor-reactive, IgG alloantibodies are present in most long term allograft recipients. Additionally, neither the amount, nor the IgG subclass of donor-reactive alloantibody correlates with graft acceptance or with the development of TVS or fibrosis. Thus, donor-reactive alloantibodies are not indicative of either graft acceptance or rejection, nor are they indicative of TVS development.