With the proliferation of immunoassays used in the clinical laboratory, we have come to realize that there are many factors which can interfere with these assays. Some of these, like exogenous radioactivity, will only interfere with one type of immunoassay--the radioimmunoassay. Other interferences will affect all types of immunoassays, although perhaps to differing degrees. This article will survey some of the more common types of interfering substances which may affect immunoassays, including tube types, serum factors, and the interferences which are specifically due to the type of label used in the assay.