Praziquantel, (-)- is oxopyrazinoisoquinoline derivative and a levorotated isomer of Praziquantel patented by Merck Patent G.m.b.H. as anthelmintics agent. In rabbits infested with S. japonicum, the therapeutic effect of Praziquantel, (-)-was greater than that of dl-praziquantel as rated by the number of the worms in tissues and by liver damage. Histopathological examination showed that liver egg granulomas in the levo-praziquantel group were fewer in no. and smaller in size and were predominantly composed of Pseudotubercles instead of eosinophilic abscesses. Levo-praziquantel is therapeutically superior to praziquantel, while dextro-praziquantel is almost ineffective.