SummaryHigh-dose aprotinin is now routinely used in cardiac surgery to reduce postoperative blood loss and transfusion requirements, although several cases of anaphylactic reactions to the proteinase inhibitor have been reported. As part of a multi-centre study to evaluate the immunological response to aprotinin after first exposure 61 cardiac surgical patients were treated with the Hammersmith regimen. Patients with previous aprotinin exposure were excluded from the study. To determine specific IgG and IgE antibodies blood samples were taken pre-operatively, within 3 to 4 weeks and 6 to 7 months after operation. Determinations were made by using Western Blot and ELISA methods. Fifty-six patients were followed up for a 6-month period, 26 (46.4%) of them developed IgG antibodies to aprotinin determined by Western Blot, whereas only 14 (26.8%) patients with IgG antibodies were found by the ELISA. IgE antibodies were not found in any of the patients. On hospital admission and 6 months post-operatively additional intradermal prick tests were performed. No clear-cut positive reaction to the skin test was found in any patient.