Water extracts of the bark of Uncaria tomentosa, a vine indigenous to South America, has been used for generations as an "immuno modulator". To understand the basis of this immuno modulatory effect we fed mice in their drinking water with C-Med 100, which is a commercially available water extract from Uncaria tomentosa. We found a dose-dependent increase in spleen cell numbers in the supplemented mice, but the proportions of B cells, T cells, NK cells, granulocytes, and memory lymphocytes were normal. However, there were no detectable changes of the lymphoid architecture of the spleen even after long-term treatment. Further, when C-Med 100 treatment was interrupted the cellularity returned to normal level within four weeks. The increased number of lymphocytes was most likely not due to increased production because C-Med 100 did not have any significant effect on precursor cells nor on the accumulation of recent thymic emigrants in the spleen. We conclude that accumulation is most likely due to prolonged cell survival, because adoptive transfer experiments demonstrated that C-Med 100 treatment significantly prolonged lymphocyte survival in peripheral lymphoid organs, without increasing their proliferation rate. Since the accumulation was reversible and without detectable pathological effects, these results suggest the use of C-Med 100 as a potential agent for clinically accelerating the recovery of patients from leukopenia.