To establish the incidence and timing of red cell allo-immunisation following transfusion, pretransfusion and serial post-transfusion samples were screened for allo-antibodies in a total of 452 patients who had undergone elective surgery. Antibody screening was performed by 2-stage papain, manual polybrene and indirect antiglobulin techniques (IAT). Red cell allo-antibodies were found in 42 patients and 38 of these (8.4% overall) demonstrated antibodies only after transfusion; 76% of them had Rh specificity. This rate of red cell allo-immunisation is higher than what would be expected if transfused patients were tested only once post-transfusion, as has been the case in several previous studies. For the type of patients studied, this finding may not be of clinical relevance at present because most patients undergoing elective surgery do not require further transfusion in their lifetime. However, this is changing with the longer life expectancy of the population and the increased probability of repeat surgery. Twenty-two (58%) of the antibodies were initially detected by the 2-stage papain and/or polybrene techniques, when the IAT was negative, although later 19 became positive by IAT; this added sensitivity of techniques other than the IAT, to detect early allo-immunisation may be relevant in pretransfusion testing to prevent haemolytic transfusion reactions in patients requiring repeated transfusions.