Ruthenium complexes appear to counteract the growth of lung metastases of solid metastasising tumours. This effect, is studied in detail with some derivatives of ruthenium(II) and ruthenium(III) characterised by the presence of at least one dimethylsulfoxide ligand. These compounds do not share with cisplatin the cytotoxicity for tumour cells, nor that for healthy tissues of the host. Conversely, they markedly reduce the growth of lung metastases of solid metastasising tumours of the mouse and significantly prolong the life-time expectancy of the tumour bearing hosts. Thus, ruthenium complexes show the unique property of selective inhibition of the disseminated tumour without the typical side-effects that characterise the use of anti-cancer agents. An interesting correlation between reduction of the proteolytic activity in the tumour mass, increase of the connective barrier around blood vessels and reduction of lung metastasis formation seems to point in favour of a mechanism of anti-metastatic effect related to the alteration of the tumour-host interactions. Ruthenium complexes are therefore a new and innovative tool for investigating the selective pharmacological approach to the treatment of metastases of solid tumours.