Antibodies towards the ganglioside GM1[galactosyl‐N‐acetylgalactosaminyl‐(N‐acetylneuraminyl)‐galactosylglucosyl ceramide] stimulated DNA synthesis in rat thymocytes. No mitogenic stimulation was observed with the monomeric Fab fragment of anti‐GM1, suggesting that cross‐linking of the gangliosides or associated components was required for activation by these antibodies. Incubation of thymocytes with anti‐GM1 and fluorescein‐labeled anti‐rabbit IgG at 0°C resulted in uniform ring‐like or patchy staining that developed into a pronounced cap upon elevation of temperature. The cap had a characteristic uropod form, enriched with intracellular organelles. Sodium azide and cytochalasin B completely inhibited cap formation, while colchicine was without effect. These results imply a possible direct or indirect association between surface gangliosides and submembraneous cytoskeletal assemblies that control modulation of these surface components and may transmit stimuli to the interior of the cell.