Adherent HeLa 1 and non-adherent HeLa S cells were seeded onto a preexisting monolayer of WI38 fibroblasts. Both cell types were able to attach to and to spread on top of the fibroblasts. Furthermore, both cell types were able to migrate actively from these fibroblasts if they managed to contact the underlying glass support. Both cell types were capable of penetrating the monolayer, under-lapping the fibroblast cells and finally destroying the monolayer. The cells' behavior was different when the monolayer consisted of alcohol-fixed or irradiated WI38 cells. In this case spreading on top of the treated fibroblasts resembled the concentric spreading seen on glass or plastic. This was in contrast to the spreading on untreated cells where the tumor cells became more or less spindle shaped. Moreover, the HeLa cells appeared to be less mobile and destruction of the monolayer was never observed. From all this we concluded: i: HeLa 1 and HeLa S cells have more than one attachment mechanism. ii: the spreading behavior is strongly influenced by the underlying support. iii: the upper cell surface of fibroblasts supports the spreading and movement of other cell types. iv: movement of HeLa cells and consequently the destruction of the monolayer is promoted by the intact fibroblasts.