A methanol extract from seeds of Paeonia lactiflora (Paeoniaceae, peony) was found to possess different antiproliferative activities against four different human cancer cell lines: Hela, MCF-7, HepG2 and HT-29. Furthermore, five different methanol (20, 40, 60, 80 and 100 % MeOH) fractions obtained by fractionation of the methanol extract of the seeds on a Diaion HP-20 column exhibited differential antiproliferative effects against the above four cancer cell lines. Among five fractions, the 60 % MeOH fraction showed relatively lower antiproliferative activity on MCF-7 estrogen-sensitive breast cancer cell than the other cancer cell lines. Systematic separation of 60% the MeOH fraction by silica gel and Sephadex LH-20 columns led to the isolation of four known stilbenes, trans-resveratrol (1), trans-(+)- $varepsilon$ -viniferin (2), gnetin H (3) and suffruticosol B (4). The four stilbenes (1∼4) exerted differential biphasic effects on cell proliferation of MCF-7 cells in a similar manner as genistein, a soybean isoflavone used as a positive reference, in the concentration range from 1.0 to 200 $mu$M. Three stilbenes (1 ∼ 3) weakly stimulated the proliferation of MCF -7 cells at doses below 10 JIM. However, strong antiproliferative effects on MCF-7 cell were exerted by extract 1 at a dose of 200 JIM, and by 2 and 3 at doses above 25 $mu$M. In contrast, 4 inhibited the proliferation of MCF-7 cell at a dose below 25 $mu$M, but stimulated cell proliferation at concentrations of 50 and 100 $mu$M. All four stilbenes (1∼4) stimulated the proliferation of ROS 17/2.8 osteoblast-like cells in the range of 10$^{-10}$ ∼10$^{-1}$ $mu$M. Compound 1 exhibited especially potent proliferative activity, although its activity was weaker than that of genistein. Additionally, three resveratrol oligomers (2∼4) also exhibited concentration-dependently moderate proliferative activity, but less than that of 1. These results suggest that resveratrol, and its dimer and trimers from the seeds of Paeonia lactiflora may act as a phytoestrogen, but in a somewhat different manner from that of genistein.