B16 melanoma variant lines, which resisted lysis by syngeneic lymphocytes, were selected in vitro by repeated exposure of the tumor cells to purified cytotoxic lymphocytes. The resistance of the tumor cells to lysis mediated by syngeneic lymphocytes was not accompanied by a loss or masking of major histocompatibility antigens. Neither the lymphocyte-susceptible B16 (F10) nor the lymphocyte-resistant B16 (F10Lr) cells grew in allogeneic recipients, and both were destroyed in vitro by allogeneic lymphocytes. The resistance to lysis by syngeneic lymphocytes was not accompanied by loss or masking of receptors for macrophage recognition and destruction. F10Lr cells did not protect F10 cells from lymphocyte-mediated lysis in cocultivation experiments. Immunization of syngeneic mice in vivo with B16-F10 cells successfully protected mice against challenge with B16-F10 cells. However, mice immunized with B16-F10Lr cells were not protected against challenge with B16-F10 cells. B16-F10Lr cells were not immunogenic when tested under this condition. Light, scanning, and transmission electron microscopic studies of tumor cell-lymphocyte interaction suggested that the resistance of B16-F10Lr cells to destruction by syngeneic lymphocytes in vitro was due to the masking or absence of tumor-specific antigen(s) present on the lymphocyte-susceptible F10 cells.