Reactions to intravenous protamine include rash, urticaria, bronchospasm, hypotension. and/or pulmonary artery pressure elevation. We have previously shown that in diabetic patients receiving daily protamine insulin injeetions, the presence of anti-protamine IgE or IgG antibodies are significant risk factors for acute, life-threatening reactions when protamine is given intravenously. To study protamine reactions further, we measured serum anti-protamine IgE and IgG antibody levels, in-vitro basophil histamine release and intracutaneous skin testing to protamine serially in an NPH-insulin dependent diabetic who had a severe, protracted anaphlactic reaction to protamine. At the time of his protamine reaction, his serum contained 8·5 ng/ml of anti-protamine IgE and 1·3 μg/ml of anti-protamine IgG antibody. One month following the reaction both anti-protamine IgE and IgG increased to 16 ng/ml (twofold rise) and 90·5 μg/ml (70-fold rise), respectively. With time, both anti-protamine IgE and IgG antibody declined. Serial intradermal skin tests using protamine sulphate did not discriminate between the protamine reactor and nine normal control subjects who had no prior exposure nor any demonstrable serum IgE antibody to protamine. In-vitro basophil histamine release to protamine sulphate was inconclusive in discriminating between the protamine reactor and normal control subjects. We postulate that protamine may be an incomplete or univalent antigen that must first combine with a tissue macromolecule or possibly heparin to become a complete multivalent antigen capable of eliciting IgE antibody-dependent mediator release.