The chloroform-soluble fraction of the vinegar treated small black soybean [Glycine max (Leguminosae)] showed antiproliferative activity against human myeloid leukemia HL-60 cells, in terms of inhibition of proliferation and induction of apoptosis. Bioassay-guided chromatography of the chloroform-soluble fraction resulted in the isolation of two isoflavonoid compounds, genistein and daidzein, as active principles. Genistein showed more potent antiproliferative effects against HL-60 cells. Treatment of HL-60 cells with genistein induced apoptosis in a dose dependent manner. Apoptosis was judged by the detection of DNA fragmentation by a flow cytometry and the degree of apoptosis was assayed by RT-PCR.