Tumor cells traverse basement membranes during the transition from in situ to invasive carcinoma and during passage through blood vessel walls in the metastatic process. In vivo studies have shown that circulating tumor cells first attach to the exposed basement membrane. Attachment is followed by focal dissolution and penetration by tumor cell pseudopodia. No previous in vitro system has existed for studying this process at the biochemical or ultrastructural level. We therefore have prepared whole sheets of intact human amnion basement membrane in an in vitro model of tumor cell invasion.