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Although adjuvants are important components of vaccines, few studies have been conducted to establish the criteria on adjuvant selection and to investigate mechanisms of adjuvant actions during vaccination. Here we found that complete Freund adjuvant (CFA) induced a CD11b+ cell population in a B-cell independent manner. This cell population exhibited strong ability to inhibit T-cell−mediated rejection of tumor transplants. In vitro studies indicated that these cells induced T-cell apoptosis and down-regulated interferon-γ production. Nitric oxide (NO) played important roles to achieve these effects. Plenty of NO was produced by these CFA-induced CD11b+ cells. The addition of NG-nitro-L-arginine-methyl ester, an inhibitor of NO synthase, rescued T cells from apoptosis and partially abrogated the detrimental effects of CFA in cancer vaccines. Incomplete Freund adjuvant, one of the adjuvants still being used in clinical trials, also induced a similar cell population. Our results reveal a previously unknown mechanism in which the myeloid cell population induced by Freund adjuvant impairs antitumor immunity, and highlight the importance of adjuvant selection during tumor vaccination.