Immunological tests based on antigen-antibody reactions are widely used to detect and quantify tumor markers, hormones, infections, and other targets. The detection sensitivity and specificity of these tests have recently improved markedly as a result of technical advances in antibody preparation and optical meas- urement. Despite these technical innovations however, it remains impossible to completely prevent non- specific reactions. In this paper, we describe the causes of such non-specific reactions as obstacles to im- munological testing, mainly in automated testing; special reference is given to some cases of interference by cross-reactions, heterophile antibodies, and autoantibodies that we encountered while measuring carcinoem- bryonic antigens (CEAs). Since not only non-specific reactions but also exogenous factors, such as sample processing, can cause abnormal data, noteworthy points in actual test settings are also described. To cope with the abnormal data, it is necessary to raise risk awareness among technicians who perform immunological tests, and establish an information-sharing system that allows queries from physicians to be quickly relayed to staff in the actual test setting. [Review].