We have explored biochemical and hematologic parameters that might indicate acute humoral xenograft rejection (AHXR) following pig organ transplantation in baboons. Baboons (n = 15) received an immunosuppressive regimen, and underwent a miniature swine or hDAF kidney (Group 1, n = 6) or heart (Group 2, n = 7) transplantation. Control baboons (Group 3, n = 2) received the immunosuppressive regimen without organ transplantation. Blood chemistry and hematologic parameters were measured daily. Baboon and porcine cytomegalovirus were monitored. In Groups 1 and 2, organ grafts survived for up to 29 days. A plasma fibrinogen of <80 mg/dL on 2 consecutive days, and a serum lactate dehydrogenase of >600 U/L and aspartate transaminase of >300 U/L, were associated with the development of AHXR in both heart and kidney grafts. In Group 1, a decrease in platelet count of >150 000/μL within 3 days, or a count of <50 000/μL, were associated with AHXR. In Group 2, a creatine phosphokinase of >500 U/L was associated with graft failure. In Group 3, no abnormalities were observed. The possibility that porcine CMV may play a role in graft injury could not be excluded. Noninvasive parameters were identified that have predictive potential for AHXR. Monitoring of these might enable therapeutic intervention to reverse rejection.