Previous reports have demonstrated that extracts of the plant Uncaria tomentosa inhibit tumor cell proliferation and inflammatory responses. We have confirmed that C-Med 100, a hot water extract of this plant, inhibits tumor cell proliferation albeit with variable efficiency. We extend these findings by showing that this extract also inhibits proliferation of normal mouse T and B lymphocytes and that the inhibition is not caused by toxicity or by induction of apoptosis. Further, the extract did not interfere with IL-2 production nor IL-2 receptor signaling. Since there was no discrete cell cycle block in C-Med 100-treated cells, we propose that retarded cell cycle progression caused the inhibition of proliferation. Collectively, these data suggested interference with a common pathway controlling cell growth and cell cycle progression. Indeed, we provide direct evidence that C-Med 100 inhibits nuclear factor kappa B (NF-kappa B) activity and propose that this at least partially causes the inhibition of proliferation.