The cytotoxic effect and antitumor activity induced by the novel organotin compound triethyltin(IV)lupinyisulfide hydrochloride, have been investigated. Different patterns of antiproliferative effects have been observed in a panel of human tumor cell lines in vitro. Toxicity studies in mice reported acute toxicity at the doses of 21 and 17.5 mg/kg which progressively disappeared at lower concentrations. On this basis, the doses of 3.5, 7 and 14 mg/kg were selected to assess the antitumor activity in vivo against the P388 leukemic cells xenografted in mice. This compound was able to induce a dose-dependent significant reduction of tumor volume, up to 46%, at the highest concentration (p = 0.0062) without important toxicity, as also confirmed by histological analysis of the main organ tissues. This preliminary study seems to hold interest for further investigations in different tumor models as well as for the evaluation of optimal drug route and schedule.