Abstract: Novel platinum(II) complexes represented by the general formula ##STR1## in which R.sub.1 represents a lower hydrocarbon radical of 1 to 3 carbon atoms, R.sub.2 and R.sub.3 each independently represent a hydrogen atom or a lower hydrocarbon radical of 1 to 3 carbon atoms, or R.sub.2 and R.sub.3 together may form --(CH.sub.2).sub.4 --or --(CH.sub.2).sub.5 --, Y and Z each independently represent an ammonia molecule or a monodentate amine of 1 to 7 carbon atoms or X and Y together may form a bidentate diamine of 2 to 10 carbon atoms, and X.sup.- represents an inorganic acid anion or an organic carboxylate anion, and agents for treating malignant tumors which contain them as effective components. The compounds are effective against cisplatin-resistant tumors, are soluble enough for intravenous administration, are stable in aqueous solution, and have excellent antitumor activity.