We then investigated the biological activity of the AMA1specific IgGs from U.S. vaccinees and Malian children by GIA ( Figure 1 ). As in our previous study [27] , the AMA1-specific IgGs from U.S. vaccinees showed a significant correlation between antibody level and percent inhibition when they were tested against the homologous strain of parasites (Spearman rank correlation, p,0.001, r s = 0.96, 95% confidence interval (CI) 0.91-0.99 for 3D7; p,0.001, r s = 0.83, 95% CI 0.60-0.94, for FVO) and the relationship followed a symmetrical sigmoid curve when the antibody levels were log transformed (r 2 = 0.93 for 3D7 and 0.82 for FVO). The AMA1-specific IgGs from Malian children showed almost the same level of growth-inhibitory activity at the same level of antibody in the GIA well, regardless of which pools of total IgG were tested. When the data from all AMA1-specific IgGs from Malian children were combined, there were significant correlations between antibody levels by ELISA and percent inhibition in GIA (for 3D7, p,0.001, r s = 0.97, 95% CI 0.92-0.99; for FVO, p,0.001, r s = 0.94, 95% CI 0.86-0.98), and each relationship followed a symmetrical sigmoid curve (r 2 = 0.99 for 3D7, 0.94 for FVO). These results indicate that there was no obvious difference in biological activity of antibodies among AMA1-specific IgGs induced by an AMA1 vaccination (IgGs from U.S. vaccinees), by natural infection (IgGs from D0-1, 2, 3 & 4 and Hib-1, 2 & 3 pools) and by both natural infection and vaccination (IgGs from AMA1-1, 2 & 3 pools).