Natural suppressor cells from the bone marrow of intact animals and mice with Th1- and Th2-dependent immune response largely suppressed the production of Th1 (IFN-gamma, IL-2) and Th2 cytokines (IL-4). Antiproliferative activity of natural suppressor cells from intact and immunized animals was mediated by NO. Proliferation of Th1 cells was suppressed greater than that of Th2 cells. This selective effect of natural suppressor cells on the proliferation of target lymphocytes was due to higher stimulatory potential of Th1 cells (in comparison with Th2) towards natural suppressor cells.