T-cell immune responses are critical for protection of the host and for disease pathogenesis during infection with Plasmodium species. We examined the regulation of CD4(+) T-cell cytokine responses during infection with Plasmodium berghei ANKA (PbA). CD4(+) T cells from PbA-infected mice produced IFN-γ, IL-4 and IL-10 in response to TCR stimulation at levels higher than those from uninfected mice. This altered cytokine response was dependent on parasitemia. To examine the specificity of the response, mice were adoptively transferred with CD4(+) T cells from OT-II TCR transgenic mice and were infected with PbA expressing OVA. Unexpectedly, CD4(+) T cells from the OT-II-transferred wild-type PbA-infected mice showed high levels of IFN-γ production after stimulation with OVA and the cells producing IFN-γ were not OT-II but were host CD4(+) T cells. Further investigation revealed that host CD4(+) T cells produced IFN-γ in response to IL-2 produced by activated OT-II cells. This IFN-γ response was completely inhibited by anti-CD25 mAbs, and this effect was not due to the block of the survival signals provided by IL-2. Furthermore, IFN-γ production by CD4(+) T cells in response to PbA antigens was dependent on IL-2. These findings suggest the importance of IL-2 levels during infection with malaria parasites and indicate that CD4(+) T cells can produce IFN-γ without TCR engagement via a bystander mechanism in response to IL-2 produced by other activated CD4(+) T cells.