The in vitro activity of a series of ruthenium clusters, [(η6-C6H6)(η6-C6Me6)2Ru3(μ-H)3(μ3-O)][BF4], [(η6-C6H6)(η6-1,4-iPrC6H4Me)(η6-C6Me6)Ru3(μ-H)3(μ3-O)][BF4], [(η6-C6H6)4Ru4(μ-H)4][BF4]2, [(η6-C6H5Me)4Ru4(μ-H)4][BF4]2 and [(η6-C6H6)4Ru4(μ-H)3(μ-OH)][Cl]2, has been evaluated against A2780 and A2780cisR ovarian carcinoma cell lines. Both triruthenium clusters are very active compared to ruthenium compds. in general, whereas the tetraruthenium clusters do not display significant cytotoxicities. Since the triruthenium clusters are known to form supramol. interactions with arenes and other functions, it is possible that such interactions are also important with respect to their mode of biol. activity. The X-ray structure anal. of [(η6-C6H5Me)4Ru4(μ-H)4][PF6]2 is also reported. Graphical Abstr. The in vitro activity of a series of ruthenium clusters has been evaluated against A2780 and A2780cisR ovarian carcinoma cell lines and their activity compared to cisplatin. The triruthenium clusters are very active, while the tetraruthenium clusters do not display significant cytotoxicities.