Cell morphological changes induced by cytochalasin B were investigated in Ehrlich ascites tumor cells with light and electron microscopes. Cytochalasin B (10μg/ml) induced zeiotic knobs around the cell surface. These knobs gathered to one pole of the cell surface where cells were treated at 37℃ for 10min but not at low temperature and low concentration of cytochalasin B. The migration of knobs was concomitant with the grouping of microvilli and, as a result, the cell surface became smooth except for the knob grouping area. To study the behavior of cell surface molecules during the knob formation induced by cytochalasin B, cells labeled with cationized ferritins were treated with cytochalasin B. Cationized ferritins themselves induced cap formation in these cells. Zeiotic knobs formed by cytochalasin B gathered at the cationized ferritin-induced cap area. These results suggest that cationized ferritin-induced capping and cytochalasin Binduced knob migration were closely related. The linkage of cell surface molecules to the action site of cytochalasin B was also suggested. The action site of cytochalasin B was discussed in relation to the above results and in relation to experiments using Concanavalin A.