The ultrastructural characterization of the present continuous cell line, derived from a primary ductal infiltrating carcinoma (d.i.c.) of the human breast, has brought to light a remarkable morphological similarity with the original neoplastic cell population. A major parallelism is the permanent presence in culture of two categories of cells, exhibiting a strong isomorphism with the in vivo counterparts. The presence of well defined ultrastructural features, as duct-like structures, microvillous projections, junctional complexes and intracytoplasmic crypts, is a further confirmation of the breast cancer origin of this line. The significance and perspective of these findings are discussed.