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Purpose: The bacterial resistance of refrigerated and cryopreserved aortic allografts in a highly virulent infection in a dog model was studied. Methods: The infrarenal aorta of 12 dogs was replaced with either a cryopreserved aortic allograft (group I, n = 6) or a refrigerated aortic allograft (group II, n = 6) in infected sites. Allografts were harvested from dogs and stored for 1 week, either by cryopreservation (-140^oC) or refrigerated method (4^oC), in a preservation medium. At the time of implantation, induction of infection was achieved with an infected piece of knitted Dacron placed just beneath the allograft. The Dacron was contaminated in vitro by soaking it in a solution with Staphylococcus aureus PR209. All 12 dogs received no adjunct antibiotic or antithrombotic therapy. Four weeks after implantation, the animals were killed to recover the grafts for bacteriological and histological analyses. Bacterial results were expressed as colony-forming units (CFU)/cm^2 of graft material. Results: In group I, only one allograft grew bacteria at 2.16 x 10^6 CFU/cm^2, with a blood culture positive for S aureus. In group II, one dog died at 3 weeks from a false septic aneurysm rupture, all the allografts were infected (P < .05) with a mean bacterial count of 9.41 +/- 6.8 x 10^4 CFU/cm^2, and three blood cultures were positive for S aureus. The patency of the grafts was analyzed at the time of recovery. Three laminar thrombi without occlusion were present in group I; none were present in group II. A better preserved endothelium in group I was revealed by means of histologic analysis staining with factor VIII antibody before implantation. After 4 weeks of implantation in the infected site, infected allografts presented polynuclear infiltrates in the media with a high degree of inflammatory reaction, and endothelial recovery was more significant in group I, with numerous young plump cells. Conclusion: This study demonstrates that cryopreserved allografts implanted in infected sites in a dog model can produce greater bacterial resistance. (J Vasc Surg 1999;29:1090-6.)