ABSTRACT: Ascitic fluids and sera from patients with malignant tumors were tested for their ability to modulate the mitogen-induced blastogenic responses of normal subjects' peripheral blood mononuclear cells in vitro. The addition of either ascitic fluid or serum to cultures of normal blood cells greatly enhanced the blastogenic response of cells to phytohemagglutinin P, and markedly depressed the responses to concanavalin A and succinyl-concanavalin A. The blastogenic response of the cells to pokeweed mitogen was unaffected by the addition of serum and depressed by the addition of ascitic fluid. Autologous normal serum also enhanced the response to phytohemagglutinin P but had no effect on the response to the other mitogens. These activities were concentration-dependent and heat-stable (56°C, 60 min) and could be detected even if the ascitic fluid or serum was added as late as the second day of culture. Cells that had been preincubated with serum or ascitic fluid and washed well before culturing with the mitogens responded in the same manner as cells cultured in the presence of serum or ascitic fluid. The mitogen-induced blastogenic responses of mononuclear cells were not affected by the addition of autologous cells that had been preincubated with either serum or ascitic fluid, washed, and treated with mitomycin C. Indomethacin (2×10−7M) did not prevent the ascitic fluid-mediated depression of blastogenic responses of normal cells. The ascitic fluid and serum of these cancer patients appeared to contain a specific immunoinhibiting substance which exerted its effects by a direct action on the responding mononuclear cells and not by the induction of suppressor cells.