We investigated the capacity of two glial tumor cell lines (CxT24neo3 and CxT3Cl5) to invade through reconstituted basement membrane (Matrigel, MG). The purpose of our experiments was to establish whether the number of cells or the mode of malignant progression would quantitatively modify the invasion of a brain tumor cell population. To accomplish this goal, we used a vital-dye method to assess the fraction of cells that invaded through 30 micrograms MG coated on a polycarbonate filter (8 microns pore size). Our experiments demonstrated that the fraction of invasive CxT24neo3 and CxT3Cl5 cells in vitro reproducibly differed as a function of the number of initially seeded cells. This showed that invasion through MG was subject to quantitative changes caused by the number of cells present. Since CxT24neo3 and CxT3Cl5 became malignant by transfection with different oncogenes, the results also indicated that the type of quantitative change was influenced by the mode of malignant progression.