In Western blotting, it detects an antigen of 125kDa in human liver and 135kDa in tumors of histiocytic origin. Comparative study of this MAb and a standard CD68 MAb showed that their antigens are different. Its antigen in all macrophage types studied is located on the plasma membrane and within cytoplasmic structures including lysosomes. This MAb shows a restricted reactivity to cells of the monocyte/macrophage system. It specifically reacts with blood monocytes and stains resident macrophages in a wide variety of human tissues. This MAb does not stain antigen-presenting cells, e.g., Langerhans cells. Reportedly, its reactivity is restricted to histiocytes and macrophages.