Interleukin (IL)-21 modulates T-cell-associated, B-cell-associated, and natural killer cell-associated immunity. However, the potential of IL-21 to simultaneously stimulate cellular and humoral antitumor responses and the mechanisms involved have not yet been adequately explored. In this report, we examined the immune-modulating effect of IL-21 when used in vitro and its adjuvant effects when administrated concomitantly with T-cell transfer for cancer therapy. Use of IL-21 in concert with IL-2 in culture up-regulated both type 1 and type 2 cytokine production of activated tumor-draining lymph node cells and enhanced their therapeutic efficacy. Administration of IL-21 and IL-2 as an adjuvant to T-cell transfer resulted in simultaneously elicited cellular and humoral responses. This concurrent response has led to effective regression of established pulmonary metastatic tumors and s.c. tumors. T-cell transfer plus IL-21/IL-2 administration conferred systemic immunity to the treated hosts. This was evident by the induction of protective immunity against tumor rechallenge, expansion of memory T cells, and significantly elevated serum levels of IFN gamma and IL-10. Furthermore, we observed significantly enhanced tumor-associated antibody response after T-cell + IL-2 + IL-21 therapy. Cytotoxic antibody subclass IgG2b increased strikingly in the sera of treated animals; they bound specifically to MCA205 tumor cells, and such immune sera mediated tumor cell lysis in the presence of complement. Use of B-cell-deficient mice provided direct evidence that humoral responses contribute to T-cell + IL-2 + IL-21-elicited antitumor immunity. Collectively, these findings provide a rationale to evaluate the use of IL-21 in T-cell therapy of human cancers.