Fifty-nine patients receiving platelet transfusions for bone marrow failure secondary to malignancy were screened at regular intervals for the presence of antibodies to human leucocyte (HLA) and platelet specific antigens. HLA antibodies occurred in 19 patients, 10 of whom also developed platelet specific antibodies. The HLA antibodies disappeared in 10 of 15 patients followed for periods of 2-14 months. In two patients this occurred whilst still receiving platelet transfusions. Antibody reappeared in only two of six patients subsequently transfused. Antibodies to platelet specific antigens were detected in 28 patients. They were transient, often appeared in association with infection, and in 50% of cases tested demonstrated autoantibody activity. There was no association with antibiotic drug therapy, or PFA/EDTA-dependent cryptantigens. Platelet recovery at 1 h or 20 h post transfusion was not significantly reduced in the presence of platelet specific antibodies. These findings have important implications for the selection of platelet donors for alloimmunized recipients.