Pregnant women will be excluded from this study since it involves administration of medications contraindicated in pregnancy. Children less than 14 years old will be excluded because of the amount of blood required for the immunologic studies. The age of consent in Mali is 18 years of age, so children aged 14 to 17 years will sign an assent form in addition to the consent form to be signed by a parent or tutor. However, married women between the ages of 14 and 17 will sign consent as adults in view of the laws governing emancipation of women in Mali. Subjects who do not participate in this study will receive PZQ as part of the national schistosomiasis control program.