Dendritic cells (DCs) possess numerous dendrites that may be of great advantage to interaction with T cells. However, it has been poorly understood how the dendritic morphology of a DC is controlled. In the present study, using a murine spleen-derived DC line, we analyzed effects of CCR7 ligands, CCL19 and CCL21, on dendritic morphology. Mature DCs, but not immature DCs, showed vigorous migration to either CCL19 or CCL21. CCL19 also rapidly (within 30 minutes) induced marked extension of dendrites of mature DCs that was maintained at least for 24 hours. On the other hand, CCL21 failed to induce rapid dendritic extension, even though a modest dendritic extension of mature DCs, compared to that by CCL19, was induced 8 or 24 hours after treatment with CCL21. In addition, pretreatment with a high concentration of CCL21 significantly inhibited the rapid dendritic extension induced by CCL19. Thus, it is suggested that CCL19 and CCL21 exert agonistic and antagonistic influences on the initiation of dendritic extension of mature DCs. The CCL19-induced morphologic changes were completely blocked by Clostridium difficile toxin B that inhibits Rho guanosine triphosphatase proteins such as Rho, Rac, and Cdc42, but not by Y-27632, a specific inhibitor for Rho-associated kinase. These findings suggest that Rac or Cdc42 (or both), but not Rho, are involved in the CCL19-induced dendritic extension of mature DCs.