The distribution, function and ontogeny of the mononuclear phagocyte system of the chicken were characterised using the monoclonal antibody (MAb) KUL01. KUL01 specifically recognises chicken monocytes, macrophages and interdigitating cells, as well as activated microglia cells. Its tissue distribution allowed to discriminate KUL01 from all earlier described MAb, reactive with mononuclear phagocytes. The specificity of KUL01 for mononuclear phagocytes was further confirmed in functional assays: KUL01-positive macrophages in spleen and liver actively took up colloidal carbon, while monocytes and spleen and gut macrophages contained non-specific esterase and acid phosphatase activities characteristic for antigen-processing. Further, it was demonstrated that KUL01-reactive peripheral blood monocytes express MHC-II, but not CD4. In all tissues investigated, the same morphological subtypes of macrophages were detected in chicken at similar localisations as in mammals, indicating a high degree of conservation between the mononuclear phagocyte system of the chicken and of mammals.