A single intraperitoneal injection of GdCl(3)in doses of 14 or 28 mg/kg to mice with intravenously transplanted Lewis pulmonary adenocarcinoma on days 3 or 8 after tumor transplantation reduced the mean number of tumor metastases in the lungs. The effect of GdCl(3)was more pronounced if it was injected at the stage of tumor dissemination (day 8). The positive antimetastatic effect of GdCl(3)was presumably due to its capacity to macrophage depression. The direct (not mediated through mononuclear phagocyte system cells) effect of GdCl(3)on tumor cells cannot also be excluded.