The heat-labile enterotoxin of porcine E.coli isolates was assayed by plate immune hemolysis assay (PIHA) and ligation of rabbit ileum (LRI), the positive result rate by PIHA and LRI was higher than that by BSRS. The purification method was studied and modified bacteria was dealt with by polymyxin B sulfate. After being precipitated by 80 % ammonium sulfate the LT was isolated and purified through Sephadex G-100 column chromatography and identified by SDS-PAGE.