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Rabbits were infected with 48-52, 198-202 and 498-502 cercariae of Schistosoma japonicum and treated ig with single doses of praziquantel (40 mg/kg) 4 and 8 wk post infection. The worm reduction rates of the 8 wk groups were 87.9, 92.2 and 97%, respectively. These values were significantly higher than 48.5, 52.3 and 58.6% for the corresponding 4 wk groups. The serum antibody titres of the 8 wk post infection rabbits were apparently higher than those of the 4 wk rabbits. Meanwhile, for the same duration of infection, the specific antibody levels in the heavy worm burden groups were also higher than those in the light worm burden groups. The results obtained in the mice infected with S. japonicum at different intensities and treated ig with single doses of praziquantel at 300 mg/kg at different durations were similar to the results in the rabbits. The correlation between efficacy and specific antibody level was confirmed by calculating the coefficient of correlation with r values of 0.454-0.983 (P less than 0.01). The results also indicated that heavy infections with S. japonicum were still more responsive to praziquantel treatment. In addition, the effect of praziquantel on worms was closely related to the specific antibody response in the host.