Mastocytoma cells in vitro suppressed the immune response of normal syngeneic spleen cells to sheep erythrocytes. The immune response of dispersed spleen cells from normal mice in vitro was inhibited by simultaneous incubation with relatively small numbers of mastocytoma cells or a cell-free extract prepared from these cells. Inhibition occurred when diluted cell-free extracts from the mastocytoma cells or as few as 10-4 or 10-5 tumor cells were incubated with 5 million normal splenocytes. The supernatant factor from mastocytoma cells was labile to heating at 56 degrees for 30 min. Detailed examination of the physicochemical properties of the mastocytoma cell extract capable of suppressing immune responses to sheep red cells should be of value.