SummaryLevels of antibody a binding to the schistosomulum surface and b mediating in vitro eosinophil‐dependent cytotoxicity of schistosomula were studied and compared to the in vivo levels of resistance to cercarial challenge in mice infected with irradiated cercariae, unirradiated cercariae of single or mixed sex, or injected with eggs. Antibody‐binding was assessed by counting the number of IgG‐Fc‐receptor bearing cells (P388D1cells) adhering to mechanically‐transformed schistosomula. Significant levels of adherence occurred with sera taken from 1–2 weeks following exposure to irradiated cercariae, the level increasing gradually thereafter and being enhanced by repeated exposure. Sera from the bisexual infection showed a dramatic increase in binding activity between weeks 5–8, and with the single sex infections there was a steady rise up to week 10 followed by a sharp rise between weeks 10–12. Weekly injections of eggs produced a steady rise in serum binding activity. Sera taken just before challenge were also tested for their ability to mediate killing of schistosomula by eosinophil‐enriched preparations of heterologous rat peritoneal exudate cells in vitro. Significant levels of killing occurred with all sera, but the greatest lethal activity was found in sera from the egg‐injected, chronically‐infected and 200 male cercariae‐infected groups. This ranking did not correlate with in vivo resistance against challenge as assessed by worm recovery, the egg‐injected and single sex‐infected groups failing to mani