Opuntia ficus-indca(Op) extract has been claimed to have several therapeutic properties in oriental medicine including anti-inflammatory and anti-rheumatoid arthritis effects. Little is known of its effect on the activation of immune cells such as T cells and macrophages. To evaluate the functional effect of Op extract on immune cells, we examined whether Op extract stimulates the proliferation of T cells and the secretion of cytokines including IL-1 beta, IL-6 and tumor necrosis factor-alpha in THP-1 cell lines by RT-PCR. Op extract significantly enhanced the proliferation of T cell clone(D10S). Transcription of cytokines including IL-1 beta, IL-6, and TNF-alpha peaked 6 hrs after exposure to Op extract(100g/ml) in the THP-1 cell line and declined and declined thereafter. In an experiment to test the dose dependency of transcription of cytokines, transcription increased at a dose of 10 g/ml and the maximum expression was obtained at 100 g/ml, 6 hrs after exposure to Op extract. These findings suggest that Op extract is a potent stimulant of immune cells including T cells and macrophages, which acts by stimulating T cell proliferation and upregulating cytokines. These phenomena imply that some edible plants may be beneficial to living animals through the activation of immune functions.