New antitumor-active pt(II) complexes of trans-l-diamine cyclohexane containing diphosphines as a leaving group were synthesized. The structures of the pt(II) complexes were determined by analyzing the infrared and $^{31}P-nuclear$ magnetic resonance spectra. Antitumor activities of the pt(II) complexes were tested against murine leukemia $L_{l210}$ according to the protocol of the National Cancer Institute. All the pt(II) complexes Synthesized were antitumor-active. In particular, water-soluble $[pt(trans-l-dach) (DPPP)](NO_3)_2$ exhibited excellent antitumor activity, giving T/C % values of 341 and 356 respectively, each with four cured mice out of six at a dose of 25 mg/kg. These pt(II) complexes are considered to be worthy of further development.