The stimulating effect of cytosol fraction (F3) isolated from human myeloblasts (m.w. ranging from 30 to 100 kDa) and monoclonal antibody CD 3 (MEM 57) was tested on NK and LAK cell activities in peripheral mononuclear cells (PMNC) of normal donors and leukemic patients. The F3 fraction added in 10% of the total volume of RPMI 1640 medium induced significantly increased lytic activity of normal lymphocytes or IL-2 activated lymphocytes against K 562 cells in 3-day culture. Similarly, the proliferation of both cell cultures was enhanced by F3. The effect of F3 on NK and LAK cell activities in culture of PMNC from leukemic patients was less pronounced and synergic action of F3 did not occur, whereas the proliferation of leukemic cells was significantly enhanced. MEM 57 increased the cytotoxicity and cell proliferation in 3-day culture of normal lymphocytes at concentrations ranging from 10 to 250 ng/ml. MEM 57 stimulated also NK cytotoxicity estimated in freshly isolated normal lymphocytes. The deficient NK cell activity observed in PMNC of leukemic patients was induced by MEM 57 or its combination with IL-2 in 3-day culture. These observations indicate that combination of more immunomodulating agents can lead to shortening of the incubation time necessary for correction of the NK cell defective activity in PMNC of leukemic patients.