We describe an in vitro assay for measuring the ability of tumour cells to permeabilize basement membranes, using transwell chambers coated with the reconstituted basement membrane, matrigel. Unlike previous matrigel-based procedures which quantified passage of tumour cells across a matrigel barrier, the new assay measures the ability of tumour cells to degrade the basement membrane and increase the diffusion rate of fluorescent (FL) dextran through the barrier. The procedure has the major advantage that permeability can be rapidly and accurately quantified, either by fluorometry or by the use of radiolabelled dextran, thus avoiding tedious and subjective scoring methods. Optimal conditions for the assay are described. In addition, it is demonstrated that the assay can clearly discriminate between metastatic and non-metastatic tumour cell lines, metastatic tumours permeabilizing the basement membrane and non-metastatic counterparts failing to do so. A range of enzyme inhibitors suggested that the increase in basement-membrane permeability caused by the metastatic mammary adenocarcinoma 13762 MAT is probably dependent upon the synergistic action of several degradative enzymes, namely proteases, type-IV collagenase, and heparanase. Furthermore, the ability to permeabilize the basement membrane was dependent upon intact tumour cells; tumour cell extracts, lysates and supernatants were inactive. © 1992 Wiley-Liss, Inc.