Combination effects of cis-dichlorodiammine platinum (II) (DDP) paired with 12 other antineoplastic agents were investigated on a human carcinoembryonic antigen producing colon carcinoma cell line in vitro. DDP was effective in increasing the killing efficiency of many different drugs. Supraadditive effects were noted with hydroxyurea, bleomycin, cis-acid, BCNU, adriamycin and mitomycin C, and a marked synergistic effect was noted with ara-C. DDP displayed simple additive effects wit 5-fluorouracil (5-FU), Ftorafur and methotrexate, and subadditive effects in simultaneous combination with vincristine. In view of its powerful cytotoxic effects on this particular cell line and of the supraadditive effects of its combinations with nitrosoureas and mitomycin C, DDP appears as a potentially useful antineoplastic agent for combination therapeutic regimens for human colon carcinoma.