In isogeneic (sarcoma 1 in A/JAX mice) and allogeneic (sarcoma 180 in C57BL/6 mice) mouse tumor systems, treatment of the tumor-bearing mice with niridazole, an antiparasitic drug, known to be a potent suppressor of cell mediated but not humoral immunity caused enhancement of metastases to regional popliteal nodes. Niridazole also inhibited tumor growth in vivo, as manifested by a significant decrease in the weight of the primary tumors. The enhancement of metastases is attributed to the suppression of cell mediated immunity by the drug, but the mechanism of tumor growth inhibition is not yet clear.