Serum interferon of grass carp ( Ctenocpharyngodon idellus ) , which was induced by the haemorrhagic virus was purified by means of DEAE..Sepharose anion exchane chromatography, Sephacryl S..200 gel chromatography, high..performance liquid chromatography and gradient polyacrylamide gel electrophoresis. The purif ied grass carp interferon exhibited single component when analysed by the SDS polyacrylamid gel electrophoresis. It was a glycoprotein with the molecular weight of 38kD and the isoelectric point of 5. 25. Biochemical studies showed that the grass carp interferon was not sedimented at 100 000g for 2 hours, exhibited stability to heat of pH2~ 10, and resisting to DNase and RNase, but was sensitive to trypsin, Glucosidase and NaIO4. It was able to inhibit different kinds of virus in the cells of grass carp, but failed in human, mammalian, avian and shellf ish cells. The results showed that the grass carp interferon was specif ic to fish cells and viruses. Its characterization was consistant w ith that of high vertibrate ../ .. interferon.