The toxicity and antitumor activity of the novel organotin compound triethyltin(IV)lupinylsulfide hydrochloride (IST-FS 29), administered by the oral route, have been evaluated against three transplantable murine tumor models: P388 lymphocytic leukemia, B16F10 melanoma and 3LL Lewis lung carcinoma. Mild and reversible signs of acute toxicity such as behavioral symptoms, weight loss and histological alterations were mainly reported at the highest single dose of 28 mg/kg. Conversely, lower concentrations of compound ranging from 7 to 21 mg/kg did not result in major toxic effects, even after repeated dosing. The antitumor activity studies showed that fractionation dosing, rather than single bolus administration, over 1 week, might prove more active and better tolerated by allowing the achievement of the highest therapeutic total dose of IST-FS 29 (42 mg/kg). Indeed, repeated administrations of IST-FS 29 resulted in marked significant improvement of antitumor activity against B16F10 (50% of tumor volume inhibition, p = 0.0003) and, to a greater extent, 3LL (90% of tumor volume inhibition, p = 0.0001) tumors. These results indicate that IST-FS 29 might be a suitable candidate as an orally administrable anticancer drug and support its further development in human tumor xenografts.