Basing on Cotchin's identification of the so-called «interposed cells » of mammary tumours with myoepithelia, a careful analysis of the literature concerned with the nature of these elements was carried out. Data supporting the hypothesis that these cells, under hormonal unrestrained influence, may be able to acquire in part characters of embrional plasticity, and to become pluripotential giving origin to some benign or malignant neoplasms (especially of the breast) are compared, and the peculiar histological and histochemical characters allowing the identification between interposed cells and myoepithelia are considered. Results seem to be rather probative. Among the benign tumours of the breast, the « fibrosing adenosis » is of particular interest, both for the close relations of its evolution to the hormonal metabolism and for the role that the myoepithelia play in its evolution. The personal observation of over 60 cases of fibrosing adenosis permitted to check the observations related to other neoplasms as concerns the argyrophilia and histochemistry of these elements. Results are tabulated. The present research is preliminary to a more specific investigation on the role of myoepithelia in the genesis of mixed tumours of the parotis which, according to some authors, would be a hormone target organ.