A group B recipient of a group 0 kidney developed severe intravascular haemolysis due to the formation of anti-B. Immunoglobulin allotyping of donor serum, recipient serum and 'unexpected' anti-B antibody showed the antibody to be of donor origin. The patient and donor genotypes were Gm 3;5/3;5 and Gm 1,2;21/1;21, respectively, and the anti-B antibody allotype was Gm 1;21. The group B recipient of the other donor kidney showed no evidence of haemolysis. Possible factors influencing the occurrence and severity of post-transplantation haemolysis are discussed. The production of anti-A or anti-B antibodies in non-group 0 patients who receive group 0 organ transplants is well described [1-8]. We report a case of severe intravascular haemolysis in a group B patient who received a group 0 kidney, together with immunoglobulin allotyping studies which show conclusively that the antibody responsible for the haemolysis was of donor origin.