The in vitro activity of a series of ruthenium clusters, [(eta(6)-C6H6)(eta(6)-C6Me6)(2)Ru-3(mu-H)(3)(mu(3)-O)][BF4], [(eta(6)-C6H6)(eta(6)-1,4-(PrC6H4Me)-Pr-i)(eta(6)-C6Me6)Ru-3(mu-H)(3)(mu (3)-O)][BF4], [(eta(6)-C6H6)(4)Ru-4(mu-H)(4)][BF4](2), [(eta(6)-C6H5Me)(4)Ru-4(mu-H)(4)][BF4](2) and [(eta(6)-C6H6)(4)Ru-4(mu-H)(3)(mu-OH)][Cl](2), has been evaluated against A2780 and A2780cisR ovarian carcinoma cell lines. Both triruthenium clusters are very active compared to ruthenium compounds in general, whereas the tetraruthenium clusters do not display significant cytotoxicities. Since the triruthenium clusters are known to form supramolecular interactions with arenes and other functions, it is possible that such interactions are also important with respect to their mode of biological activity. The X-ray structure analysis of [(eta(6)-C6H5Me)(4)Ru-4(mu-H)(4)][PF6](2) is also reported