Cultures of CBA T6T6 mouse embryo cells were transformed by 20-methylcholanthrene (MC) treatment in vitro. Untreated and MC treated cells and reexplanted cells of tumours originating from MC treated cells were compared by scanning electron microscopy (SEM). The 3 cultures showed considerable differences in the situation of the cells compared to each other, as well as in the number and shape of the surface formations.