Background/Objective: Substance P (SP) has been reported to have immunoregulatory properties including effects on many of the mediators involved in anti-tumor immunity. In this study, we investigated the effect of SP on tumor development in a murine model of melanoma. In addition, we examined the role of natural killer (NK) and T cells in SP-mediated modulation of tumor growth. Materials and Methods: Mice were implanted with mini-osmotic pumps that delivered a continuous infusion of either SP or PBS over a 14-day period. Five days following implantation, animals received K1735 melanoma cells and tumor growth was monitored. The role of NK and T cells in SP-mediated protection was examined by antibody depletion studies. To determine if cells from SP-treated animals could delay tumor growth in animals in the absence of exogenous SP infusion, splenocytes from mice treated with SP were adoptively transferred into SCID mice. Results: In vivoSP treatment led to a significant delay in tumor growth. When animals were depleted of NK or T cells, this protective effect was lost. Adoptive transfer of cells from SP-treated animals led to a significant protective effect on tumor growth in SCID mice. Conclusion: Pretreatment of mice with SP provides protection against K1735 tumor growth, and this protection requires both T cells and NK cells. SP-mediated tumor protection can be transferred by the adoptive transfer of cells from SP-treated animals into animals that do not receive exogenous SP. These studies suggest a model in which in vivo SP treatment prior to tumor challenge primes immune mediators to prevent or delay tumor establishment.