The 1:2 adducts of Ag(I) and Au(I) with 1,2-bis(di-n-pyridylphosphino)ethane (dnpype) for n=2, 3 and 4 have been synthesised and solution properties characterised by multinuclear NMR spectroscopy. The complexes are hydrophilic analogs of the lipophilic Au(I) antitumour complex [Au(dppe)2]+ and the degree of hydrophilicity depends critically on the position of the N atom in the pyridyl ring. The complexes of d3pype and d4pype are simple monomeric [M(d3pype)2]+ and [M(d4pype)2]+ species which have a much higher water solubility than the 2-pyridyl complexes which crystallise in the solid state as dimeric [{M(d2pype)2}2]2+. In solution these 1:2 M:d2pype species exist as equilibrium mixtures of monomeric, dimeric and trimeric (Ag) or tetrameric (Au) clusters. The Au(I) and Ag(I)pyridyl phosphine complexes have been evaluated for antitumour activity against a panel of cultured human ovarian carcinoma cell lines. The results show both potent and selective activity for the compounds with IC50 values ranging from 0.18 to 1500 卮 There is a correlation between the degree of antitumour selectivity and the octanol/water partition coefficients with the greatest selectivity (500-fold range) found for the most hydrophilic complex [Au(d4pype)2]Cl. Clinical development of the parent compound [Au(dppe)2]+ was halted by liver toxicity and the hydrophilic pyridylphosphine analogs are significantly less toxic than [Au(dppe)2]+ when exposed to isolated rat hepatocytes. Convenient synthetic routes to the bidentate pyridyl phosphines d2pype, d3pype and d4pype are also described.