A number of new ruthenium compounds have been synthesised, isolated and characterised, which exhibit excellent cytotoxicity against a number of different human tumour cell lines including a defined cisplatin resistant cell line and colon cancer cell lines. Addition of hydrophobic groups to the ruthenium molecules has a positive effect on the cytotoxicity values. Evidence is provided that, after incubation of a ruthenium compound with a 46 mer oligonucleotide duplex and subsequent nuclease treatment, ruthenium is bound to a guanine residue.