Tricyclodecan-9-yl-xanthate (D609) was identified as an agent that caused selective killing of tumor cells by an unknown mechanism of action. We report an inhibition by D609 of basement membrane collagen biosynthesis in the chick chorioallantoic membrane system in vitro. In the same system in vivo D609 inhibits angiogenesis. Also treatment of rats bearing Walker 256 carcinoma with D609 results in a dose-dependent antitumor effect. These results indicate that basement membrane synthesis may be a target for developing anti-angiogenic compounds with antitumor properties.