The determination of reactants of inflammation frequently is interfered by rheumatoid factors (RF) which are able to bind with the Fc-part of immunoglobulins. False positive results might be due to such interference. These interference could not be observed between avian (IgY) and mammalian antibodies. Based on that observation anti C-reactive protein (CRP) antibodies raised in chickens and rabbits, respectively, were compared by means of a latex-agglutination test or an enzym-immuno-assay (EIA) with respect to false positive results. A total of 70 patients sera with a defined content of RF was tested. It could be shown that using the mammalian latex test 39 sera were found to be CRP-positive in contrast to 22 positive values measured by the avian test. A similar result was obtained comparing results obtained from both the avian EIA and the mammalian EIA, respectively. Based on these data the substitution of one mammalian antibodies by an avian antibody may improve tests systems if a reduction in false positive results is concerned. That might be valid not only for CRP but also for further reactants of inflammation which are determined by use of antibodies.