The effect of single dose of praziquantel (400 mg,kg) or two doses on two consecutive days was studied in mice infected with S. mansoni. With the single dose there was improvement in the serum protein pattern, towards pre-infection values & a transient rise in serum. transminases which soon declined to normal. Improvement was more evident after using two doses of praziquantel (P < 0.001). Worms were eradicated by 70%. 77.1% and 80% after the single dose and by 88%, 92% and 95% after two doses. Granuloma size was reduced by 50%, 54% and 6C% after single dose and by 58% and 73% respectively after 10, 20, and 30 days from treatment.