Bis-platinum complexes having the formula ##STR1## wherein A is a ligand having two pairs of platinum-coordinating acidic groups which are each independently a carboxylate, phenolate, sulfonate or phosphonate group, each said pair being capable of coordinating with a platinum ion to form a 5-10 membered chelate ring; Z is a water-solubilizing group; n is 0 or a positive integer, with the proviso that n is 0 only when at least one of said platinum-coordinating acidic groups is sulfonate or phosphonate or when at least one platinum ion is a Pt(IV) ion; and R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are each independently H or a C.sub.1-20 alkyl, aryl, aralkyl, or alkaryl group, or together form one or more C.sub.1-30 alkylene, cycloalkylene, arylene, aralkylene or alkarylene groups; or a pharmaceutically acceptable salt thereof, are useful as readily water-soluble antitumor agents.