Tumor-related angiogenesis is likely to be a potential target for the treatment of cancer. One key to develop this angiostatic strategy would be to find useful angiogenesis inhibitors. Here we report the effects of radicicol, a microbial angiogenesis inhibitor that we previously identified using the chorioallantoic membrane assay, and its novel analog, 14,16-dipalmitoyl-radicicol, on tumor angiogenesis and growth. As expected for agents containing a penolic hydroxyl group, systemic administration of radicicol had little or no effect on neovascularization triggered by a M5076 mouse tumor cell line or a RMT-1 rat mammary carcinoma cell line established from autochthonous rat mammary tumors induced by 7,12-dimethylbenz[a]anthracene in a mouse dorsal air sac assay system. The agent did not show growth-inhibitory activity against either transplantable M5076 tumors or autochthonous 7,12-dimethylbenz[a]anthracene-induced rat mammary tumors. In contrast, 14,16-dipalmitoyl-radicicol potently suppressed tumor angiogenesis and growth in these experimental models. Furthermore, the analog significantly prolonged the survival rate of M5076-implanted mice. Although not stronger than radicicol, it dose-dependently inhibited embryonic angiogenesis in the chorioallantoic membrane assay, the dose required for half-maximal inhibition (ID(50)) value being 23 microg (27 nmol) per egg, and showed concentration-dependent antiproliferative activity against microvascular endothelial cells in vitro. These data suggest that 14,16-dipalmitoyl-radicicol is a promising antitumor agent with antiangiogenic activity.