Background: This study compares the pathologic condition of delayed xenograft rejection in Gal-positive and Gal-knockout cardiac xenografts after pig-to-baboon heterotopic cardiac xenotransplantation when the induced anti-Gal antibody response is unregulated, blocked, or absent. METHODS.: Baboon recipients of Gal-positive, CD46 pig hearts were treated with an αGal polymer (group 1; n=11) or Gal-specific immunoapheresis (group 2; n=8) to block anti-Gal antibody. Gal-knockout cardiac xenografts recipients (group 3; n=5) received no anti-Gal therapy. Perioperative and interim biopsies were examined and antibody responses were determined. Results: No hyperacute rejection was seen and histologic findings were similar across the groups. All groups showed vascular antibody deposition in perioperative and interim biopsies and in explant samples. A prominent antibody response was detected only in group 2. Complement activation was evident by C3d deposition but deposition of C5b and C5b-9 was limited. Earliest evidence of myocardial injury was myocyte vacuolization in the absence of microvascular thrombosis or coagulative necrosis that developed later. Histology of explanted hearts exhibited mainly microvascular thrombosis and coagulative necrosis with little evidence of interstitial hemorrhage or edema. Conclusions: The histology of rejection seemed independent of the anti-Gal or non-Gal immune response. Myocyte vacuolization seems to be an early feature of delayed xenograft rejection presaging more classic pathologic features. © 2011 by Lippincott Williams & Wilkins.