Spleen cells from C57BL/6J or CBA mice inoculated iv with spleen cells from BALB/c mice produced a strong nonspecific cytotoxic effect on target cells (mouse L-cells). Lymph node cells from CBA or C57BL/6J mice inoculated sc with BALB/c spleen cells also destroyed L-cells. Lymph node cells from mice inoculated with syngeneic spleen cells were not cytotoxic. The cytotoxic effect was observed ion of allogeneic but not syngeneic spleen cells. This effect was considerably reduced or completely suppressed after partial or total removal of plastic-adherent cells.