An interferon factor was detected in grass carp ( Ctenopharyngodon idellus) serum, induced in vivo with phytoagglutinin by cell pathological effect inhibition technique. Study on its biochemical and biological characteristics showed that it was different f rom the interferon-A/B induced by virus. It was sensitive to heat ( 56 e ) , pH2 and 0. 1% SDS, and its activity could not be neutralized by the antibody against interferon-A/B. It proved that the inducer was mitogen but w ith non-virus character, and the production could be modulated signif icant ly by the phorbol myristate ester and interleukin-2. All these were consistent with the interferon-Cin human, which indicated it was interferon-Clike activity. The induction was influenced by the temperature, inducing time and dosage of phytoagglutinin. The activity was higher at the temperature of 25 e or 30 e and at phytoagglutinin dosage of 0. 5- 0. 8 mg per f ish than that at 8 e or 15 e and at that of 0. 1- 0. 3 mg per fish. The inducing time when the activity was the highest was 3 days at high dosage of phytoagglutinin, which was shorter than that of 5 days at low dosage. The interferon induction could be promoted by the phorbo l myristate ester and interluekin-2.