Inhibition by lymphoid cellular factors of the immunological cytotoxicity against a syngeneic tumour.

Lymphoid cells of C57BL mice bearing a syngeneic sarcoma are cytotoxic against the same tumour cells in vitro at two different periods of the growth of the tumour separated by a "mute period". Without additional antigenic stimulation the lymphoid cells from the "mute period" can be made cytotoxic by preincubation in vitro and elimination of adherent cells by a passage through a glass bead column. Either of these treatments alone is inefficient. The cytotoxicity revealed in this way is due to cells with different properties than those that are directly cytotoxic during the 4th week of the tumour growth. The significance of these results is discussed. It is suggested that some other cells have to be eliminated because they compete with or inhibit the activity of the "mute period" lymphoid cells made cytotoxic by preincubation.