Pseudo-allergic reactions (PAR) are caused by a variety of drugs, of particular interest by acetylsalicylic acid (ASA) and other nonsteroidal antiinflammatory drugs. The clinical symptoms often resemble immediate type hypersensitivity reactions and consist of bronchospasm, urticaria, angioedema and even anaphylactic shock. Antigen specific immune mechanisms, however, are not involved. In general, skin tests are not reliable and the diagnosis of PAR is based mainly on risky provocation tests. Therefore, the purpose of this study was to establish procedures for in vitro diagnosis of PAR to ASA. A controlled study was performed including patients with PAR to ASA based on history and positive oral provocation test and non-atopic as well as atopic controls. In this in vitro study the production of sulfidoleukotrienes (sLT) by isolated leukocytes was measured by cellular allergen stimulation test (CAST), which is based on detection of LTC4, LTD4 and LTE4 by a monoclonal antibody. Accordingly, the direct effect of ASA as well as the modulatory effect of ASA on C5a-induced production of sLT in leukocytes in vitro was investigated. In patients with PAR to ASA, C5a-induced generation of sLT was significantly increased as compared to normal controls. In contrast, there was no difference in the spontaneous release of sLT in vitro in patients and controls. Preincubation of leukocytes with ASA did not exert a significant modulatory effect on the spontaneous or the C5a-induced production of sLT in patients and controls. In summary, the present study provides a novel in vitro test system for the diagnosis of PAR to ASA by measurement of sLT release in leukocytes.(ABSTRACT TRUNCATED AT 250 WORDS)