A mouse monoclonal IgG2a antibody, M195, with reactivity restricted to early myeloid cells, acute non-lymphoid leukemia cells (ANLL), and monocytic cells is described. The antibody was derived from a mouse immunized with live human leukemic myeloblasts. Specificity of binding of mAb M195 was determined by protein-A red blood cell rosetting assays, immunoabsorption, radioimmunoassays with iodine-125 labeled M195 IgG and F(Ab)'2, and complement cytotoxicity with live human cells and cell lines representing a broad range of lineages and tissues. Antigen expression was restricted to myeloid and monocytic leukemia cell lines and a fraction of mature adherent monocytes. Mature myeloid cells, T and B cells, erythrocytes, and platelets were negative. The antigen was not expressed on adult human tissues in immunoperoxidase and immunofluorescence assays. Blocking antigen was not found in the serum of patients with ANLL. Ten thousand sites per cell were expressed on myeloid or monocytic leukemia cell lines and 5000 sites per cell on mature monocytes. M195 IgG bound to its antigen target with an avidity of 3 x 10(9) liters/mol and induced rapid modulation of the antigen. M195 IgG was able to effectively kill cells with rabbit or guinea pig complement, but not human complement. The antibody did not mediate antibody dependent cellular cytotoxicity. The molecular nature of the target antigen remains unknown but it appears to be carried on the CD33 protein p67. Because of its restricted distribution on myelomonocytic cells, mAb M195 may be useful in studying myeloid differentiation, in the clinical diagnosis of ANLL, in purging of bone marrow of ANLL, and/or in monoclonal antibody therapy in vivo.