The effects of the treatment of tumor cells of MCa mammary carcinoma and TLX5 lymphoma with the ruthenium complex Na[trans-RuCl4(DMSO)lm] for several transplant generations were studied on tumor growth and metastases formation. On TLX5 lymphoma cells, treatment was performed in vitro prior to in vivo inoculation of tumor cells in intact or immunesuppressed mice. Either considering tumor take and growth or its capacity to invade the brain of the inoculated hosts, Na[trans-RuCl4(DMSO)lm] did not induce any significant modification. Conversely, in mice with MCa mammary carcinoma, the in vivo treatment of tumor cells in immunesuppressed hosts caused a progressive increase of DNA activity and, starting from the 4th transplant generation, a significantly increased susceptibility of lung metastasis formation to a further treatment in intact mice. These data seem to suggest that Na[trans-RuCl4(DMSO)Im] does not induce chemical xenogenization of tumor cells nor its repeated treatment induces resistance in tumor cells. Conversely, it appears that Na[trans-RuCl4(DMSO)lm] may select a tumor cell population which maintains its capacity to metastasise to the lung but with enhanced sensitivity to the antimetastatic properties of this compound.