Human peripheral blood mononuclear cells were stimulated with pokeweed mitogen (PWM) to study the role of mononuclear phagocytic cells (MNP) in lymphocyte proliferation. MNP were identified by cytoplasmic alpha-naphthyl acetate esterase, by the capacity of phagocytosis and by lysozyme synthesis. It appeared that after 3 days of stimulation with PWM all MNP disappeared from the cultures and remained absent during prolonged culture in mitogen-free medium. In non-stimulated cultures MNP remained. The disappearance of MNP from cell cultures was caused by a lymphocyte-derived factor, which was transferable by cell-free supernatants of stimulated mononuclear cells. From experiments in which cultures were treated with different concentations of PWM and from pre-culture experiments, it could be shown that in vitro lymphocyte proliferation required both non-stimulated lymphocytes and freshly prepared MNP. In addition, the decreasing concentration of PWM as stimulating agent, resulted in a decreasing proliferation of lymphocytes, which was inversely proportional to the presence of MNP.