A mouse monoclonal antibody (mAb), designated as mNI-11, was produced. The reactivity of mNI-11 was assessed by immunofluorescence assay with various cells. Myelomonocytic cell line, U937, and lipopolysaccharide (LPS)-treated U937 (LPS-U 937 cells) were found to bind strongly to the mAb. EBV-B cell line and peripheral blood mononuclear cells (PBMCs) were found to bind moderately to it. This mAb strongly induced homotypic cell aggregation of LPS-U937 cells. The mAbs to CD 18 and CD 54 completely inhibited the LPS-U 937 cell aggregation induced by mNI-11. The surface antigens of the U 937 and LPS-U 937 cells recognized by mNI-11 had a molecular size of 95-97kDa as determined by immunoblotting analysis.