Schistosomiasis is characterized clinically by an increase in eosinophil counts in bone marrow and peripheral blood. This eosinophil response is also manifest pathologically by cell accumulation around the invading schistosomula and the parasite ova retained in host tissues. The parasites undergo a series of antigenic and maturational changes within the host that result in two distinct peaks of eosinophilia. Furthermore, products of parasite interaction with complement, antibodies, and sensitized T lymphocytes have been shown to enhance eosinophil migration and chemotaxis. Eosinophils, which constitute a prominent cellular component in the host response to the schistosomes, may be activated biochemically. Both in vivo and in vitro studies have delineated a protective role for the eosinophils against schistosomula and eggs. The ability of these cells to destroy multicellular parasites has been shown to be due to highly reactive oxygen reduction products or to release of cationic or major basic proteins from the granules.