There are relatively few monoclonal antibodies (MAbs) to rat monocyte/macrophages available. We describe here 2 new such antibodies. The first, 109.2, recognizes most rat monocyte/macrophages and all polymorphs. The antigen recognized by this antibody is upregulated by 15 mins exposure to PMA (Phorbol myristate acetate) but down regulated by overnight exposure to LPS (lipopolysaccharide). It is probably an adhesion molecule and is likely to represent the rat equivalent of CD11b. The second antibody, 112.1, recognizes lysozyme in rat macrophages, particularly alveolar macrophages. In addition it also recognizes lysozyme in hen, rabbit and human macrophages. It also recognizes lysozyme in other tissues such as Paneth cells and proximal renal tubular cells.