OBJECTIVE:To study the effective portion of the ultrafiltration membrane-separated ethanol extract of Pae-onia lactiflora and its antineoplastic activity in vitro. METHODS:The content of peoniflorin in ethanol extract of P.lactiflora was determined by HPLC. The drug-containing serum of mice was used to cultivate tumor cells such as HT29,HL60 and S180,then the inhibitory effect of P.lactiflora ultrafiltrate on the tumor cells and the association between dosage and administration time of the P.lactiflora ultrafiltrate were observed. RESULTS:P.lactiflora(10～100 g?kg-1) showed marked inhibition on all kinds of the above-mentioned tumor cells in a dose-dependent manner; the drug-containing serum also showed marked inhibition on all of kinds of tumor cells at different time in a time-dependent manner. CONCLUSION:The effective portion of ultrafiltration membrane-separated ethanol extract of P.lactiflora has antineoplastic activity in vitro.