Our recent Phase 2 trial in Malian children showed that there was a small, but significant, increase of growth-inhibitory activity when data from all AMA1-immunized children was analyzed (the GIA was performed with total IgGs in the study) [28] . The level of increase in the activity was not different from the level observed in U.S. adults who were immunized with the same vaccine formulation [28] . In the U.S. vaccine trial, all of the volunteers showed negligible levels of inhibition before vaccination, and after immunization the growth-inhibitory activity of total IgG was a function of anti-AMA1 titer. However, in the Malian children, the increase of growth-inhibitory activity was observed only in AMA1immunized children who had less than 20% inhibition before vaccination. None of the children who had more than 20% inhibition before vaccination (14/89 children in the AMA1immunized group) showed more than a 20% increase, while antibody levels measured by ELISA increased in most of the children. On the other hand, out of the remaining 75 children with less than 20% inhibition before vaccination, 25 (33%) children had more than a 20% increase after vaccination. In the present study we wished to investigate the effect of non-AMA1 IgGs in the children to determine whether similar interference effects could be identified. Therefore, we made multiple pools of total IgG, separated them into AMA1-specific and non-AMA1 IgGs, and tested them by GIA. The non-AMA1 IgGs showed an interference effect on growth-inhibitory activity.