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. Copyright © 2011 John Wiley & Sons, Ltd.