Previous studies in our laboratory have shown that DBA/2 mice injected ip with syngeneic P815 tumor cells transfected with the HLA-CW3 gene (P815-CW3) showed a dramatic expansion of activated CD8+ CD62L− T cells expressing exclusively the Vβ10 segment. We have used this model to study the regulatory mechanisms involved in the development of the CW3-specific CD8+ response, with respect to different routes of immunization. Whereas both intradermal (id) and ip immunization of DBA/2 mice with P815-CW3 cells led to a strong expansion of CD8+ CD62L− Vβ10+ cells, only the id route allowed this expansion after immunization with P815 cells transfected with a minigene coding for the antigenic epitope CW3 170–179 (P815 miniCW3). Furthermore, depletion of CD4+ T cells in vivo completely abolished the specific response of CD8+ CD62L− Vβ10+ cells and prevented the rejection of P815-CW3 tumor cells injected ip, whereas it did not affect CD8+ CD62L− Vβ10+ cell expansion after id immunization with either P815-CW3 or P815 miniCW3. Finally, the CW3-specific CD8+ memory response was identical whether or not CD4+ T cells were depleted during the primary response. Collectively, these results suggest that the CD8+ T cell response to P815-CW3 tumor cells injected ip is strictly dependent upon recognition of a helper epitope by CD4+ T cells, whereas no such requirement is observed for id injection.