Skin-tests, patch-tests, Prausnitz-Küstner reactions, passive cutaneous anaphylaxis in guinea pigs according to Ovary and lymphocyte proliferation tests in vitro using thymidine 2-C14 have been carried out in 31 patients with aspirin intolerance. Acetylsalicyclic acid, aspiryl-polylysine, lysine acetylsalicylate, and sodium salicylate were used in these investigations. Collagen-induced platelet aggregation according to BORN in the presence of aspirin was also studied. An immunological mechanism was revealed in only a small percentage of the patients studied, indicating in the remaining cases the possible role of nonimmunological processes or the inadequacy of the test technique employed.