Human serum albumin (HSA), coupled to filter paper discs, was used in the radioallergosorbent test (RAST) to evaluate nonspecific binding which results from high levels of total serum IgE. Binding of 125I-labelled anti-IgE to HSA coated discs was significantly correlated with total IgE levels. A significantly better correlation was seen, however, using HSA chemically treated under alkaline conditions prior to coupling with the disc. Use of HSA as a control for individual serum samples in the RAST is discussed.