Previously we reported that a mega-dose of Vitamin C enhanced the initial stage of delayed-type hypersensitivity reaction in Balb/c mice. In this study its effects were further evaluated as follows. Mice were administered Vitamin C intraperitoneally at 0.625 mg/day or at 5mg/day for variable days before, during, or after being sensitized with DNFB. T cells were isolated in each group and examined. When stimulated antigen-specifically or non-specifically in vitro, mice showed elevated thymidine uptake and a shift of cytokine secretion profiles toward Th1, i.e., elevated levels IL-2, TNF-alpha, and IFN-gamma, and lowered level of the Th2 cytokine IL-4, only when Vitamin C was administered during sensitization. T cells from those mice administered Vitamin C before sensitization or after challenge showed the same T cell properties as those from PBS-treated mice. Mice were also given 0.625 mg/day of Vitamin C during primary and/or secondary immunizations with KLH and secondary specific antibody titers in sera were measured. The total specific antibody titer was lowered in Vitamin C-treated animals whenever treatments were administered, and this was entirely attributed to decreased levels of IgG1 and IgE antibodies. Based on these results, we suggest that an exogenously administered mega-dose of Vitamin C shifts immunity in Balb/c mouse toward Th1 and that these affects occur only when Vitamin C is administered during T cell activation.