Six hybridoma clones were obtained secreting monoclonal antibodies (Mab) against differentiating antigens of human hemopoietic cells. ICO-1 Mab detect Ia-like (Dr) antigens. Mab react with B-lymphocytes and monocytes without detecting antigens on granulocytes and T-cells. Antigen expression was enhanced following cell activation in a blast cell transformation test and mixed lymphocyte culture (MLC). ICO-1 Mab blocked MLC. The molecular weights of the antigen were 29 and 34 kilodaltons. Comparative studies of ICO-1 Mab with other Mab against Ia-like antigens revealed their identical reactivity. ICO-11 Mab detect the antigen on 4% of blood lymphocytes, 75% thymocytes, monoblasts, and CFU-GM. These Mab block NK-cell activity of blood mononuclear cells. ICO-GM-1 Mab detect the antigen on myelomonocytic cells and their precursors, but not CFU-GM. These Mab block binding of the C3Bi complement component to CR3 receptor and NK-cell activity. ICO-G2 Mab detect the antigen expressed at final stages of granulocyte differentiation. ICO-10 Mab detect the antigen on early thymocytes and ICO-02 on undifferentiated blast cells. Mab were shown to be applicable for human leukemia and lymphoma immune diagnosis.