Establishment of the invasion assay and comparison of the invasive potential of different tumor cell types: A. In order to assay differences in the invasive potential of different types of tumor cells, the tumor cells are seeded at day 0 on top of a gel composed of collagen. Following cell adhesion and spread a second layer of collagen is seeded on top of the cell monolayer. The gels are subsequently cultured between 3 and 9 days and invasion of cells from the monolayer into the collagen assayed as described in methods. B. The invasive properties of MDA-MB-435 melanoma cells, MDA-MB-231 and LM2-4 breast cancer cells and HT1080 fibrosarcoma cells were compared. Following the seeding of the cells, the invasion of cells from the monolayer into the upper and lower collagen gel was measured in intervals of 3 days. Representative low power (10×) pictures of perpendicular gel sections in which the tumor cells are stained with eosin-Y are shown.