Local delivery systems that provide sustained, high concentrations of antitumor cytokines in the tumor microenvironment while minimizing systemic dissemination are needed to realize the potential of cytokine-based immunotherapies. Recently, co-formulations of cytokines with chitosan solutions have been shown to increase local cytokine retention and bioactivity. In particular, intratumoral (i.t.) injections of chitosan/IL-12 can eliminate established tumors and generate tumor-specific immune responses. In the present study, we explored the mechanisms by which chitosan potentiated IL-12's antitumor activity. The location of chitosan/IL-12 injection was found to be critical for optimal cytokine delivery. I.t. injections eliminated 9 of 10 MC38 adenocarcinomas while contralateral and peritumoral injections delayed tumor growth but could not eliminate tumors. Microdosing studies demonstrated that IL-12 depots, simulated through daily i.t. injections with IL-12 alone, were not as effective as weekly i.t. chitosan/IL-12. 50-75% of mice receiving daily IL-12 microdoses and 87.5% of mice receiving weekly chitosan/IL-12 were cured of MC38 tumors. Chitosan was found to increase IL-12-mediated leukocytic expansion in tumors and tumor-draining lymph nodes (TDLNs) by 40 and 100%, respectively. Immunophenotyping studies demonstrated that chitosan co-formulation amplified IL-12-induced increases in important effector populations, such as CD8(+)IFN-γ(+) and NKT cells, in tumors and dendritic cell populations in TDLNs. Remarkable increases in Gr-1(+)CD11b(+) tumor infiltrates were also observed in mice receiving chitosan or chitosan/IL-12. This population does not appear be suppressive and may facilitate the local antitumor response. Presented data suggest that chitosan-mediated depot formation and enhanced local cytokine retention is significantly, but not entirely, responsible for increased cytokine bioactivity.