In experiments designed to test why high levels of antibody-dependent, eosinophil-mediated killing of schistosomula are routinely observed in this laboratory, several factors that may contribute to variations in eosinophil activity were examined. The most important factors were: (1) the source of eosinophils, with marked variation being demonstrated not only, as previously shown, between individuals, but also between different cell preparations from a single individual; (2) the serum used as a source of anti-schistosomulum antibodies and (3) the age of the schistosomula at the time of assay. In contrast, addition of fresh normal serum as a source of complement had a relatively slight effect when the killing assay was carried out in round bottomed tubes. A more marked enhancement was observed in flat bottomed microtitre plates, and it is suggested that this enhancement may be attributable to the release of chemotactic complement components. No difference was observed between a laboratory maintained and a recently derived isolate of Schistosoma mansoni, either in initial susceptibility or in loss of susceptibility after 3.5 h of culture. In contrast to the marked effects of eosinophils under most conditions tested, there was no evidence for extensive neutrophil-mediated damage under the same conditions.