The surface morphology of Chinese hamster ovary cells treated with cytochalasin B (CB) has been examined using the scanning electron microscope. The cells respond to treatment with CB by retracting peripheral processes, rounding up, and assuming a smooth or gently convoluted surface. This response occurs within minutes. Cells in different stages of the cell cycle all respond in a similar manner. When CB is removed from treated cells by washing with conditioned medium, the cells regain their normal surface conformation within minutes. The surface topography of these released cells is characteristic of their stage in the cell cycle. Because CB causes an alteration in the morphology of the cell surface and because of the speed of the response and recovery, it is proposed that the primary site of action of CB is the cell surface.

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