The modulation of the metastatic progression of breast cancer has been evaluated in vitro and in vivo with RAPTA-T, [Ru(η6-toluene)Cl2(PTA)], an organometallic ruthenium compd. In vitro RAPTA-T inhibits some steps of the metastatic process such as the detachment from the primary tumor, the migration/invasion and the re-adhesion to a new growth substrate. All these effects are boosted when cells grow on components of the extra cellular matrix such as collagen IV and fibronectin and minimized on the non-specific substrate poly-L-lysine and are more pronounced when expts. are performed with the highly invasive MDA-MB-231 cells than with the non-invasive MCF-7 or the non-tumorigenic HBL-100. In vivo RAPTA-T selectively reduces the growth of lung metastases, an effect that might be explained by the in vitro activity. The effect on tests requiring the interaction of the tumor cells with extra cellular matrix components, might suggest an interaction with cell surface mols. in the activity of this ruthenium compd.