Absence of CD4(+) T cell help has been suggested as a mechanism for failed anti-tumor cytotoxic T lymphocytes (CTL) response. We examined the requirement for CD4(+) T cells to eliminate an immunogenic murine fibrosarcoma (6132A) inoculated into the peritoneal cavity. Immunocompetent C3H mice eliminated both single and repeat intraperitoneal (IP) inoculums, and developed high frequency of 6132A-specific interferon-gamma (IFNgamma)-producing CTL in the peritoneal cavity. Adoptive transfer of peritoneal exudate cells (PEC) isolated from control mice, protected SCID mice from challenge with 6132A. In contrast, CD4 depleted mice had diminished ability to eliminate tumor and succumbed to repeat IP challenges. Mice depleted of CD4(+) T cells lacked tumor-specific IFNgamma producing CTL in the peritoneal cavity. Adoptive transfer of PEC from CD4 depleted mice failed to protect SCID mice from 6132A. However, splenocytes isolated from same CD4 depleted mice prevented tumor growth in SCID mice, suggesting that 6132A-specific CTL response was generated, but was not sustained in the peritoneum. Treating CD4 depleted mice with agonist anti-CD40 antibody, starting on days 3 or 8 after initiating tumor challenge, led to persistence of 6132A-specific IFNgamma producing CTL in the peritoneum, and eliminated 6132A tumor. The findings suggest that CTL can be activated in the absence of CD4(+) T cells, but CD4(+) T cells are required for a persistent CTL response at the tumor site. Exogenous stimulation through CD40 can restore tumor-specific CTL activity to the peritoneum and promote tumor clearance in the absence of CD4(+) T cells.