Schistosomula of Schistosoma mansoni were shown to be rapidly killed in vitro by the combined effects of rat neutrophilic leukocytes, IgG antibodies from schistosome-infected rats, and heat-labile factors found in normal serum. A microassay system was developed for characterizing and quantitating this opsonization reaction. Following a single infection, serum opsonin levels reached a peak at approximately 6 weeks and then decreased only slightly until the 18th week post-infection. In the presence of antiserum of high titer, 100% of schistosomula were heavily ensheathed and apparently killed by neutrophils within 20 hours. Ensheathed schistosomula stained heavily with nitroblue tetrazolium dye, indicating the release of lytic enzymes by the neutrophils. Both ensheathment and killing were prevented if normal serum was used in place of antiserum, or if neutrophils were removed from the leukocyte suspension.