Antibodies derived from egg yolk offer many advantages over mammalian antibodies in several aspects. Chicken antibodies do not activate the human complement system and are sometimes a more suitable choice in designing solid-phase immunometric assays than mammalian antibodies. The material often recommended for immunological assays is serum. A freshly drawn serum sample contains an active complement system, which is inactivated during storage. Mammalian antibodies used in most immunological assays may activate the human complement system. Activated complement components will bind to the antibodies thereby partly block the antibody binding epitopes. We show that an active complement system in undiluted samples reduce the absorbance values by approximately 50 % when using goat antibodies but not when using chicken antibodies. This difference will cause erroneous test results that will vary depending on the handling of the samples. Chicken antibodies can be used to eliminate this interference problem.