Despite the presence of preformed antibodies against AB oligosaccharide epitopes on the donor vascular endothelium, approximately one-third of ABO-incompatible organ allografts are not rejected by a humoral mechanism. With the growing immune-manipulation of the recipient, survival rates can be raised considerably, although they remain significantly inferior to those of ABO-compatible transplantation. Data from the Collaborative Transplant Study indicate a 1-year graft survival rate of approximately 50-60% following cadaveric ABO-incompatible kidney, liver or heart transplantation, compared with 70-80% for an ABO-compatible organ. The results for infants and young children, however, are very much better than those of adults, particularly for liver and heart transplantations, and the data suggest that B-cell tolerance can develop in the infant age group. We here review clinical and experimental experience with ABO-incompatible organ and bone marrow allotransplantation and address the mechanisms by which organs or cells survive in the presence of natural anti-carbohydrate antibodies.