This review summarizes the personal experiences of the authors and their colleagues during ten years of field and laboratory studies on human immunity to Schistosoma mansoni infections. There is evidence for the very slow development with age of an acquired resistance to reinfection (demonstrable after chemotherapy of the primary infection) distinguishable from a lack of reinfection due to reduced exposure. The implications of this immunity for the design of chemotherapy programs targeted at infected schoolchildren are discussed. Observational studies on the mechanisms of immunity have demonstrated a marked helminthocidal capacity of eosinophils. Subsequent correlative studies have indicated a role of IgM and IgG2 "blocking" antibodies in maintaining the continued susceptibility of young children, and of IgE antibodies in mediating protection in older individuals. Some problems in studying human immunity, and the implications for vaccine development, are also discussed.