Antigen expression determined by ICO-11 monoclonal antibodies was studied on leukemia cells from bone marrow, blood cells from patients with lymphoproliferative diseases and natural killer (NK) cells responsible for natural resistance. ICO-11 monoclonal antibodies were shown to recognize the antigen expressed on NK-cells, predecessors of T-cells (thymocytes), myelomonocytes (myeloblasts, monoblasts) and to block NK-cell activity.