The effects of bio-active peptide (BP) from buckwheat pollen and its synthetic analogues on murine and human lymphocytes and HL-60 cells were examined. Both natural and synthetic BP had the same effect in the lymphocyte blast transformation test. The proliferative response of HL-60 cells, co-cultured for 72 h with BP, was measured by the MTT method. BP had a stimulatory effect on HL-60 cell proliferation. The optical density (OD) value was 0.339+/-0.015, whereas, that of the control without BP was significantly lower (0.279+/-0.037; P < 0.05). The CD25 (IL-2Ralpha chain) positive cells of BP-stimulated human lymphocytes assessed by indirect immunofluorescence were 53.97+/-7.12% (1 microg/ml) and 42.53+/-4.4% (10 microg/ml), significantly higher than the control (11.77+/-1.88%; P < 0.001). The content of sIL-2R in the supernatant after culture with BP was determined by ELISA and was 1782.3+/-78.18 U/ml, lower than control 2381.2+/-99.9 U/ml. Taken together, our results showed that BP has an immunostimulatory effect on lymphocytes. The likely mode of BP action and the possibility of developing a novel immunomodulator are discussed.