-When thioguanine-resistant V79 cells are introduced into spinner flasks containing V79 multicell spheroids, the mutant cells attach to the surface of the spheroids. The composite spheroids thus formed consist of external, thioguanineresistant (TGr) and internal, thioguanine-sensitive cells. Cell sorting with a Becton Dickinson FACS II was used to determine the relative position of TGr cells an...