We have observed that acute myocardial infarction is associated with the immunological response characterized a transient rise of serum immunoglobulin E (IgE). We wondered whether this reaction was specific for myocardial infarction or whether it reflected a more generalized phenomenon, perhaps triggered by tissue injury. We, therefore, made a large prospective study on patients undergoing various surgical procedures. These were the patients undergoing coronary artery bypass graft, who did (n = 39) or did not (n = 42) develop perioperative myocardial infarction, patients subjected to thoracic operations (n = 33) patients having cholecystectomy (n = 17) or repair of the inguinal hernia (n = 18) and 30 healthy volunteers forming the control group. Blood samples were drawn before the operation and then after the operation at 8, 16, 24, 48, 72, 120, 168 and 216 hours. In all samples, concentrations of serum immunoglobulins E were determined on blinded specimens by an automated microparticle enzyme immunoassay while immunoglobulins G, A and M were determined using nephelometry. In all groups studied, except the control group, serum IgE began to rise shortly after the operations reached a peak by the fifth day, and then gradually declined. This behaviour of IgE serum levels was in striking contrast to that of the remaining serum immunoglobulins G, A and M which showed a rapid fall after surgical interventions, followed by a gradual return to the initial values. The stimulation of hypothalamus-adrenal axis and release of glucocorticosteroids, interacting with de novo synthesized interleukin-6, could explain this newly observed phenomenon of the immunoglobulin E response to the tissue injury. Behaviour of serum IgE, as described by us, bears much resemblance to that of acute phase proteins. In conclusion, we hypothesize that immunoglobulin E may act in human organism as an acute phase protein.