The present ultrastructural study has been undertaken in order to contribute to the problem of morphological heterogeneity in the ductal infiltrating carcinoma of the human breast. In spite of the well known topographical variability of scirrhous breast cancers, the comparative analysis of 12 primary tumours has brought to light some basic phenotypical expressions of the neoplastic cell population. The major observation is the occurrence of two main categories of cells, which are interpretable as the transformed counterparts of the dark and light lumenal cells of the normal mammary epithelium. The phenotypical identity of the two categories has been assessed by in vitro cultivation (parallel paper). Many aberrant morphological variants, attributable to the two cell types, were observed at the tumour-stroma interface. We have therefore suggested that the high level of morphological heterogeneity may, at least in part, be the result of stromal influences on the gene expression of the transformed cells.