The intermediate filament cytoskeleton of stromal myofibroblasts from a series of twenty-eight infiltrating ductal breast carcinomas was examined by transmission electron microscopy (TEM) and indirect immunofluorescence (IF), the latter using antibodies to desmin, vimentin and prekeratin. Three cases of fibromatoses, selected as an additional source of myofibroblasts, were processed in the same manner. Stromal myofibroblasts from invasive ductal breast carcinomas rich in actin and readily identified by IF, were most numerous in the "young" edematous mesenchyme, areas corresponding to early stromal invasion or the peripheral invasive cellular front. Within the central sclerotic zone wherein clusters of neoplastic epithelial cells were surrounded by abundant collagen, most stromal cells corresponded by TEM to fibroblasts. In like fashion, myofibroblasts were most numerous in cellular, poorly collagenized portions of fibromatoses. By IF the only detectable intermediate filament protein of myofibroblasts in these two settings was vimentin. Since the appearance of stromal myofibroblasts appears to be associated with stromal invasion by malignant epithelium, their development by modulation of pre-existent periductal fibroblasts is postulated. With the exception of vascular smooth muscle cells and endothelial cells, the only periductal mesenchymal cells shown to contain vimentin were fibroblasts. The lack of desmin in myofibroblasts constitutes evidence against an origin from vascular smooth muscle cells. Because the molecular markers (intermediate filament proteins) of stromal cell differentiation presented quantitative but not qualitative modifications, the transformation of fibroblasts into myofibroblasts is quite likely, suggesting that myofibroblasts may be more closely related to fibroblasts than to smooth muscle cells.