A stable cell line (MCF-7), derived from a pleural effusion of a patient with metastatic breast carcinoma, was maintained in these laboratories for more than 3 years in conventional monolayer culture. To further characterize the tumor origin of the MCF-7 line, cells were grown on collagen-coated cellulos sponges. On the three-dimensional sponge matrix, the cells formed clusters, ductlike structures, and lumina similar to the patterns observed in the antecedent primary tumor and in the pleural metastasis. The similarity between the original tumor and the cells grown in sponge suggested that the MCF-7 cells did in fact retain the potential to express the histologic patterns of tumor, even in the absence of stroma support. This study confirmed the utility of sponge culture for the investigation of the retention of tumor characteristics by cultured cells of neoplastic origin.