The presence of constitutively produced interferon (IFN)-alpha in the blood of healthy individuals has been the subject of contradictory discussions for years. Immunologic as well as biologic test procedures have demonstrated striking differences regarding serum IFN-alpha under physiologic conditions. We investigated the presence of immunoreactive IFN-alpha in serum samples of 923 healthy blood donors by means of a widely used commercially available ELISA. Of these, 254 (27.5%) exhibited detectable serum IFN-alpha levels. The sera of 85.1% of these people also contained IFN-beta. Both IFN were also demonstrated in EDTA-anticoagulated plasma. However, none of these samples exhibited any antiviral effect on human A549 lung carcinoma cells challenged with encephalomyocarditis virus. Samples with high IFN-alpha ELISA activity did not abolish the antiviral action of added natural IFN-alpha, thus excluding IFN-alpha inhibitory factors. The experiments suggest that the detected compounds probably did not represent IFN-alpha but were the result of a cross-reaction with unknown serum components. A variety of disorders has been associated with elevated serum IFN-alpha levels that in most cases were detected by ELISA. In view of our data, these findings need to be carefully reevaluated. For the purpose of monitoring IFN-alpha levels in therapy of atopic, autoimmune, or malignant disorders, an appropriate detection system for IFN-alpha is advisable.