To test the biological activity of peptide-specific antibodies, the latter were purified by affinity chromatography using three serum pools obtained from Papua New Guinean adults. The 3 serum pools were first tested in ELISA assays against 21 peptides that were the most antigenic (Table 1) ; from these, 18 peptide-specific antibodies were purified from the most positive serum pool and tested again in ELISA. These 18 antibodies all reacted with parasite native proteins in infected red blood cells as shown by IFAT ( Figure 1A ; Table 2 ). Reactivity was restricted to blood stages, since the antibodies did not react with sporozoites stages (data not shown), and this reactivity was also peptide-specific as shown by IFAT competition assays with the corresponding peptide ( Figure 1A ).