A novel mixed $NH_3/NH_2OH$ platinum(II) complex cis-[$Pt(NH_3)(NH_2OH)Cl_2$] was synthesized and characterized by elemental analysis, FAB-MS, FT-IR and $^1H$ NMR spectroscopy. This complex was determined to have a good water-solubility and satisfactory stability. The pertinent complex was evaluated for its in vitro cytotoxicity against 3AO, HCT-116, LNcap, A549/ATCC and SGC-7901 human carcinoma cell lines. It shows appreciable cytotoxic activity that is comparable with cisplatin and is much more active than carboplatin.