Previous work has demonstrated that dendritic/tumor fusion cells induce potent antitumor immune responses in vivo and in vitro. However, little is known about the migration and homing of fusion cells after s.c. injection or the kinetics of CD4+ and CD8+ T cell activation. In the present study, fluorescence-labeled dendritic/MUC1-positive tumor fusion cells (FC/MUC1) were injected s.c. into MUC1-transgenic mice. The FC/MUC1 migrated to draining lymph nodes and were closely associated with T cells in a pattern comparable with that of unfused dendritic cells. Immunization of MUC1-transgenic mice with FC/MUC1 resulted in proliferation of T cells and induced MUC1-specific CD8+ CTL. Moreover, CD4+ T cells activated by FC/MUC1 were multifunctional effectors that produced IL-2, IFN-gamma, IL-4, and IL-10. These findings indicate that both CD4+ and CD8+ T cells can be primed in vivo by FC/MUC1 immunization.