Several years ago it was reported by us that some patients on penicillin therapy, when transfused with donor blood containing hemagglutinating penicillin antibody, suffered an untoward reaction similar to anaphylaxis or serum sickness. We are currently screening all NIH donors for penicillin hemagglutinating and IgE antibody and doing a followup on all patients receiving such units. Eventually we will be able to document the possibility of such an occurrence and the role it plays in transfusion reactions.