Several aspects of the behaviour of polyoma virus-transformed BHK cells in culture have suggested that they are more deformable than BHK cells. This possibility was tested by applying negative pressure at the cell surface by means of a micropipette. It was found that PyBHK cells in early mitosis are twice as deformable as BHK cells in the same stage. In addition, the taut, non-ruffling margins of both cell types when fully spread are much less deformable than the extending, ruffling leading lamella. The degree of deformability of these cells is correlated with the distribution and organization of microfilaments and consistent with this, deformability increases greatly in the presence of cytochalasin B. The significance of deformability studies such as these is discussed.