Invasion of brain by tumor cells is an inherent feature of the malignant phenotype. Assays to quantitate invasiveness should provide a powerful tool to investigate this phenomenon. We have developed a modified in vitro assay to measure tumor cell invasion, attachment, and chemotaxis using a barrier of the complex basement membrane Matrigel on gelatin-coated filters. Within 5 hours, 7.8% of U251MGp and 2.6% of SF126 human malignant glioma cells invaded the Matrigel and filter, compared with 0.8% of normal human leptomeningeal cells. The extent of invasion was directly proportional to incubation time and filter pore size and inversely proportional to the Matrigel concentration. Cells from exponentially growing U251MGp cultures invaded more readily (10.9%) than cells from plateau-phase cultures (2.3%); however, labeling studies with bromodeoxyuridine showed that quiescent cells and rapidly dividing cells were equally capable of invading. This suggests that the mechanisms underlying invasion by malignant glioma cells are distinct from those underlying proliferation and indicates the need for therapy aimed specifically at invasive behavior. In a practical application of this assay to test a potential anti-invasive strategy, monoclonal antibodies to the beta subunit of an integrin receptor mediating attachment to the extracellular matrix inhibited invasion by U251MGp cells in a dose-dependent manner. This assay should allow evaluation of the cellular and molecular basis of brain tumor progression and perhaps aid the development of rationally designed drugs that limit tumor invasion. It may also allow prediction of the clinical behavior of neoplasms in individual patients.