Treatment of lymphoid malignancies with monoclonal antibodies (mAbs) and immunoconjugates is a promising new immunotherapeutic approach. However, few published studies have examined in detail the subcellular fate of antibodies following binding to lymphocyte cell surface antigens. In this study, we have investigated the disposition of monoclonal anti-CD3 antibody 64.1 following binding to normal and malignant T-lymphocytes by using cellular radioimmunoassays and immunoperoxidase and immunogold electron microscopy. Anti-CD3 mAbs were predominantly cleared from the cell membrane at 37 degrees C by receptor-mediated endocytosis, although passive shedding of antibody was also observed. Internalized antibody was sequentially transferred from coated pits to receptosomes and eventually to lysosomes. Intralysosomal degradation appeared to be the ultimate fate of internalized radiolabeled mAbs and was followed by exocytosis of free 125I to the culture medium. Ammonium chloride and monensin were potent inhibitors of lysosomal degradation of 125I-anti-CD3 mAbs and caused intracellular trapping of radiolabeled antibodies. The rapid endocytosis, degradation, and exocytosis of antibody observed in these studies elucidate the mechanism of the improved efficacy of anti-CD3 immunoconjugates when used in conjunction with inhibitors of lysosomal action.