Cultured together epithelial cells of mouse kidney (line MPTR) were seen to form a mixed cohesive sheet. The mutual interaction of different cells in the sheet was studied by phase contrast microscopy, time-lapse microcinematography, scanning electron microscopy and by some other morphological tests. The upper surface of the mixed sheet was continuous on the border of the two cell types. Despite this border between different cell types was continuously moving from FBT towards MPTR, as if MPTR was "pushed aside" by FBT. In some cases such a moving caused a total exclusion of MPTR from the sheet. Excluding cells differed from the excluded ones in their better spreading and better adhesion to the underlying substrate. Possible mechanisms of such an exclusion are discussed.