Abstract: Objective To explore the killing effect of different fractional proteins from Microtus fortis （Mf） serum to S. japonicum juveniles， and to find possible association of the proteins with the natural resistance to schistosome infection. Methods The proteins in Mf serum were separated by means of ion-exchange column chromatography and molecular sieve column chromatography. After desalted by dialysis and lyophilized， the proteins were dissolved in DMEM medium which contained 300 U/ml penicillin and 300 μg/ml streptomycin, and the two-day old schistosomula were added in for in vitro cultivation (100±20/well). The killing activity of the fractional proteins to the juvenile worms was defined by mortality rate. Results 58 fractional proteins were separated from Mf serum， in which six proteins were confirmed to have a significant killing activity to schistosomula. The mortality of schistosomula all reached 37% and above， and the highest mortality（87.5%） was observed in the fraction 18.1， while the negative control was 25.0%（P＜0.01）. Conclusion Some fractional proteins in Microtus fortis serum show an effect in the natural resistance to schistosome infection.

Key words: Microtus fortis, Schistosome japonicum, Schistosomula, Fractional protein, Killing activity