In our study of the human monocyte response to migration inhibitory factor (MIF), we screened murine anti-monocyte monoclonal antibodies (MAb) for their ability to affect the MIF response. When blood monocytes were preincubated for 30 min at 37 degrees C with MAb of various cell specificities, it was found that Mo3e-MAb could inhibit the monocyte response to MIF. This effect was observed at an antibody concentration of 0.37 microgram/10(6) monocytes. This blocking activity appeared to be specific for Mo3e-MAb, since MAb specific for Mo1, Mo2, Mo4, alveolar macrophage (PAM-1), and platelet (Plt-1) antigens were unable to block the MIF response. Specificity was also shown by experiments performed with MAb similar to Mo3e-MAb in their immunofluorescence staining patterns. Two MAb of this type were not able to block the MIF response. All of these MAb are of the IgM isotype, and therefore it is unlikely that the blocking activity of Mo3e-MAb is mediated by the Fc receptor. These studies suggest that Mo3e-MAb may represent an anti-MIF receptor antibody. Mo3e-MAb recognizes a protease-sensitive antigen of 80 kD and 50 kD that is weakly expressed on fresh blood monocytes but is up-modulated by stimulation of monocytes with bacterial lipopolysaccharide, muramyl dipeptide, or phorbol compounds. Mo3e is also expressed by human myeloid cell lines, U-937 and HL-60, that have been stimulated in culture by biologically active phorbol diester compounds.