Human peripheral lymphocytes that have been exposed to phytohemagglutinin (PHA) release mitogenic factors (MF) on subsequent incubation in fresh medium. In this investigation, MF-rich supernatants were analyzed for their ability to induce DNA synthesis in various fractions of peripheral human lymphocytes. MF was found to induce higher DNA synthesis in T-cell preparations that in unfractionated lymphoid preparations or B-cell-rich fractions. Evidence is presented showing that some types of non-T-cell can inhibit the MF response of T cells. These cells are nonadherent and nonphagocytic and require DNA synthesis to express their inhibitory activity. These results suggest that the regulation of the action of lymphocyte-derived MF is cell-mediated.