A triterpene acid mixture consisting of oleanolic, ursolic and betulinic acid isolated from a standardized rose hip powder (Rosa canina L.) has been shown to inhibit interleukin (IL)-6 release from Mono Mac 6 cells. The present study examined the effects of the triterpene acid mixture on the cytokine production and proliferation of CD4⁺ T cells and CD19⁺ B cells induced by a self-antigen, human thyroglobulin and by lipopolysaccharide in cultures of normal mononuclear cells. The triterpene acid mixture inhibited the production of tumor necrosis factor-α and IL-6 with estimated IC₅₀ values in the range 35-56 µg/mL, the Th1 cytokines interferon-γ and IL-2 (IC₅₀ values 10-20 µg/mL) and the antiinflammatory cytokine IL-10 (IC₅₀ values 18-21 µg/mL). Moreover, the mixture also inhibited CD4⁺ T-cell and CD19⁺ B-cell proliferation (IC₅₀ value 22 and 12 µg/mL, respectively). Together, these data demonstrate that oleanolic, ursolic and betulinic acid are active immunomodulatory constituents of the standardized rose hip powder. However, since the estimated IC₅₀ values are in the µg/mL range, it is questionable whether the content of the triterpene acids in the standardized rose hip powder, alone, can explain the reported clinical effects.