Emodin has been reported to induce apoptosis in many human cancer cell lines, although its effects on leukocyte functions in vitro have not been demonstrated Therefore, the purpose of this study was to assess the effect of emodin on the phagocytosis of macrophages, the activity of natural killer cells and the expression of cytokines in leukocytes from Sprague-Dawley rats. Leukocytes, isolated from rats, were placed into culture plates for incubation with or without various concentrations of emodin for 1-6 hours and the functions of macrophages and natural killer cells were evaluated by flow cytometric analysis. The results indicated that emodin caused a decrease in phagocytosis of macrophages after treatment for up to 4 hours but 6-hour treatments led to an increase in the phagocytosis of macrophages. Further, emodin increased the activity of natural killer cells, both effects being dose-dependent. The levels of cytokines from the examined leukocytes were evaluated by ELISA and the results indicated that emodin increased the levels of IL-1beta and TNF-alpha, results which were confirmed by PCR assay for the mRNA expressions of the examined cytokines.