This report demonstrates that chicken anti-protein A can prevent both soluble and surface-bound Staphylococcal protein A from binding to either human or rabbit IgG. In an ELISA assay, chicken anti-protein A prevented > 98% of the soluble protein A from binding to the human IgG-Fc coat. In a blotting assay, chicken anti-protein A prevented the membrane-bound protein A from interacting with the human IgG probe. When intact S. aureus (Cowan I strain) was bound to the surface of a microassay plate, chicken anti-protein A blocked > 98% of the cell wall protein A and permitted the probing of the surface components with human IgG. In another immunoassay, rabbit anti-enterotoxin A IgG was used to measure enterotoxin A concentrations in S. aureus culture medium supernatants after soluble protein A was blocked by chicken anti-protein A. Thus, the binding of chicken anti-protein A to protein A almost completely eliminates false positive results and permits the measurement of specific antibodies or antigens in a variety immunoassays where protein A is present.