The antitumor efficacy of the vascular targeting agent combretastatin A-4 disodium phosphate (CA4DP) was evaluated in a xenograft model of Kaposi's sarcoma (KS) grown in athymic mice. Response to CA4DP alone or in combination with localized radiation treatment or systemic chemotherapy (cisplatin or vinblastine) was assessed using a clonogenic cell survival or tumor growth delay assay. Administering increasing doses of CA4DP to tumor-bearing mice resulted in a dose-dependent increase in tumor cell kill. CA4DP also enhanced the antitumor effects of radiation and chemotherapy approximately 10-100-fold. Although single doses of CA4DP as large as 300 mg/kg failed to alter tumor growth, the same total dose, administered as 3 fractions in 5 or 9 days, resulted in significant growth delay. Such repeated CA4DP exposures also significantly increased the response of KS xenografts to cisplatin. These findings suggest that CA4DP ought to be considered as a candidate agent for therapeutic evaluation in AIDS-KS patients.