Human glioma cells, growing as spherical colonies in agarose gel, or as monolayers on glass or plastic, were studied with time‐lapse cinematography and electron microscopy. The cells in the agarose cultured colonies often had ruffling‐like membrane structures which were similar in for, although smaller in size, than those observed on monolayer cultured cells. The ruffling‐like structures were more frequent at the periphery than in the central regions of the colonies which was in parallel to the proliferative pattern. In time‐lapse cinematography it was seen that pinocytotic vacuoles were formed from ruffling membranes in the monolayer cultures. In the transmission electron microscope, such vacuoles were also found near the ruffling‐like structures in the agarose cultured cells. In dense monolayer cultures, ruffling and associated pinocytosis were to a large extent transferred from the margin to the upper surface of the cells. This capacity may be an important property for the ability of the malignant cells to attain extreme high cell densities in monolayer cultures and to grow as colonies in agarose cultures. As has been previously shown, the normal counterpart of the gliomas, the glia cells, cannot grow to high densities; they do not ruffle on their upper cell surface and are unable to grow in suspension or agaros