High efficiency leukocyte reducing filters can remove more than 99.9-99.99% of white cells from cellular blood components and are considered to be effective in decreasing HLA alloimmunization of patients with haematological malignancies. A multi-institution study was performed to determine whether white cell filtration would also be effective in preventing alloimmunization in surgical transfusion recipients. Patients who were to receive red cell blood transfusions during and/or within 48 hours after surgery were randomly assigned to receive red cells/ fresh frozen plasma that had been leukoreduced using a high efficiency filter at the bedside or buffycoat-depleted red cells transfused through an aggregate filter. Of 87 patients with no alloantibodies at entry, 17% (8/47) of those in the leukoreduction group, who received a mean of 0.3 x 10(6) leukocytes as a result of their transfusions, produced lymphocytotoxic antibodies at day 14 after transfusion, compared to 5% (2/40) in the buffycoat-depleted group, who had received a mean of 1,234.2 x 10(6) leukocytes. This difference in the alloimmunization rate between the two arms was not statistically significant. Reduction of leukocytes by bedside filtration does not appear to be effective in preventing HLA alloimmunization in surgical transfusion recipients. The alloimmunized cases suggest that an indirect allorecognition pathway may be involved in the formation of anti-HLA. Further measures are needed to reduce alloimmunization of immunocompetent patients.