Macromolecular dextrans carrying substituents terminated by sulfhydryl groups or terminated by aromatic amines effectively inhibit the growth of a fibrosarcoma and of a mammary adenocarcinoma in a syngeneic mouse model. These compounds have no or very low toxicity to animals and are nontoxic to fibrosarcoma cells in vitro. Small-molecular-weight compounds carrying the same substituents as the above dextrans are without any effect on the growth of these tumors. A dextran substituted with mercury-containing side chains is growth promoting for the same fibrosarcoma in mice at doses which are nontoxic for these animals. However, the mercury-containing compound is toxic to fibrosarcoma cells in vitro. It is hypothesized that these nonpermeating macromolecules do not directly influence the tumor cells in animals but modulate the natural system of defense against tumors; cells of that system are stimulated or poisoned by the substituted dextrans.