Defining the mechanisms whereby cytotoxicity and other functions of NK cells, their subpopulations or immature precursors, are modulated by lymphokines is a prerequisite to understand the role played by these cells in physiological and pathological conditions or during adoptive immunotherapy in conjunction with biological response modifiers. We have defined an in vitro culture system in which mature NK cells, and/or subpopulations of NK and T cells present in low proportion in peripheral blood, are induced to proliferate and have purified to homogeneity a novel cytokine, operationally termed NKSF. NKSF is a potent regulator of NK cells, of which it affects functions independently from IL-2 (cytotoxicity), in synergy with it (induction of lymphokine production) or antagonizing its effects (proliferation). In order to define the molecular mechanisms that contribute to the effects of IL-2 and NKSF on NK cell functions, we propose: a) to identify lymphokines and cytotoxic molecules whose expression is differentially modulated by IL-2 in comparison with NKSF and analyze the molecular basis for their induction by the two lymphokines, b) to determine the mechanism(s) by which NKSF inhibits IL-2-induced proliferation. NKSF is not NK-cell specific. To define leucocyte subsets sensitive to NKSF, we will study its effect on T cells and some of their subsets (in particular those inducible by IL-2 to mediate MHC-non restricted cytotoxicity) and we will attempt to produce monoclonal antibodies to the NKSF-receptor. We observed that, in our culture system, cells with NK phenotype are induced to proliferate from PBL depleted of mature NK cells. It is our working hypothesis that NK cell precursors are induced to differentiate in these conditions, and we will test this hypothesis by analyzing their presence both in peripheral blood and bone marrow. From the studies proposed here, we expect to derive novel information on the biology of NK cells and on the role that distinct lymphokines play to enhance functions of these and other cytotoxic cell types.