A new invasion assay is introduced using endothelial cells grown on native human basement membrane (BM). The source of the BM was human amnion. The amnion is a uniform tissue composed of an epithelial layer resting on a continuous basement membrane overlying an avascular collagenous stroma. The epithelium was removed exposing the basement membrane (BM) surface. Human umbilical cord endothelium or bovine capillary endothelium were cultivated on the BM surface. Human squamous carcinoma cells were inoculated onto the BM surface in the presence or absence of the endothelial monolayer. Tumor cells attached readily to both the endothelial monolayer or the BM surface alone. Tumor cells which invaded the basement membrane and underlying collagenous connective tissue were collected on a Millipore filter applied to the opposite side of the amnion. Tumor cells invaded the devitalized amnion connective tissue in the absence of endothelium. The presence of either bovine or human endothelium significantly reduced the rate of tumor cell invasion. This system should be useful for further quantitative studies of the interaction between endothelium and tumor cells with regard to the mechanism of invasion.