Résumé : The MXT mouse transplantable mammary cancer is characterized by significant amounts of steroid hormone receptors. Moreover this model, as other solid tumors, contains diverse subpopulations of neoplastic cells. In the present study we have examined this pattern as well as the influence of exogenous physiologic doses of estradiol on tumor cell subpopulations. Their identification was undertaken at the morphological level employing incubation of tritiated actinomycin D with fixed material followed by autoradiography. This semiquantitative cytochemical method has proved itself to be a useful tool in order to assess the cellular heterogeneity of carcinoma cells.