Abstract: Rabbits infected with Schistosoma japoncium for 3 to 10 weeks were treated intragastrically with praziquantel at a single dose of 40 mg/kg. In the 3-week group worm reduction rate was only 14.3%, but increased to 46.1% in the 4-week group. When infected rabbits were dosed 5～10 weeks after the infection the therapeutic effect was enhanced apparently, especially in the 8 and 10-week groups. For these groups worm reduction rates of 84.2 and 87.6% were seen and most or all of the treated animals were free from female worms. These results indicated a good correlation between therapeutic effect and the emergence of specific antibody. Further studies indicate that the effect of praziquantel on 21-day old schistosomules is dependent on the presence of specific antibody in the host.