Studies of human spleen cell suspensions show that they contain a population of cells which can inhibit the mixed lymphocyte reaction. The cells appear to be able to suppress the patient's own response to an antigenic challenge in slightly greater degree than their ability to suppress a nonspecific mixed lymphocyte culture. The suppressive effect is dependent on cell dose and is linearly related to the log of cell concentration. At low dose these cells have no suppressive effect and, in fact, behave as stimulators. Exposure of these cells to an environment containing immunosuppressive drug abrogates their suppressor activity. Manipulation of these cells may prove to be of value in the control of graft rejection.