The antigen defined by MCS-2 monoclonal antibody (mAb) was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of surface and internally labeled cells. Surface iodination revealed that MCS-2 antigen on the surfaces of acute myelogenous leukemia cells, HL-60 cells, and polymorphonuclear leukocytes (PMN) had the same molecular weight (Mr 150,000) and that their autoradiographic bands were also the same. Internal labeling of HL-60 cells with [35S]methionine followed by immunoprecipitation revealed two bands whose molecular weights were 150,000 and 130,000. HL-60 cells gave stronger bands than did PMN. The intensity of the Mr 130,000 band became weaker, when internally pulse-labeled cells were cultured in the absence of labeled methionine, suggesting that Mr 130,000 glycoprotein was a precursor protein of Mr 150,000 glycoprotein. MCS-2 mAb precipitated two bands from tunicamycin-treated HL-60 cells whose apparent molecular weights were 100,000 and 110,000. When cells were cultured with MCS-2 mAb, expression of the antigen decreased rapidly (within 10 min). The degree of suppression was more prominent in PMN than in acute myelogenous leukemia and in myelomonocytic cell lines. Reexpression of MCS-2 antigen by PMN after removal of the mAb from the culture medium was not observed, but it occurred rapidly in myelomonocytic cell lines, although it was blocked by pretreatment of the cells with cycloheximide. These findings suggested that the less-marked suppression of MCS-2 antigen expression by cell lines was due to its greater synthesis by these cells. These findings suggested that MCS-2 mAb reacted with identical molecules, which were recognized by other mAbs belonging to CD13. Furthermore, modulation of MCS-2 antigen was observed by MCS-2 mAb itself.