Abstract: The tegumental surface antigens of different developmental stages of Schistosoma japonicum obtained from infected mice treated with a single oral dose of praziquantel 400mg/kg or artemether 600mg/kg were measured by indirected fluorescence assay. It was demonstrated that: (1) 4 hours after administration of praziquantel, the percentages of exposure of worm surface antigen were 90% in 2 hours' schistosomules (d0) and 25-42% in d21 to d42 schistosomules, but there was no effect on d3 to d14 old schistosomules; 16 hours later over 50% of adult worms isolated from the liver tissue showed the exposure of worm surface antigen. It is considered that the susceptibility of different developmental stages of Schistosoma japonicum to praziquantel is correlated With the exposure of worm surface antigen. (2) 1 to 3 days after administration of artemether, marked disturbance of parenchymal tissue and intestinal tubes were found in d7 schistosomules, but there was no effect on the exposure of worm surface antigen. However, 33-42% of female worms in d28 group showed worm surface antigen. It is believed that the mode of action of artemether on young and adult schistosomes is different.