To test whether the interfering effect of non-AMA1 IgGs was due to the blocking of binding between AMA1 antigen and anti-AMA1 antibody, competition ELISA was performed. A fixed amount of US-total IgG was mixed with serially diluted non-AMA1 IgGs which showed a higher interference effect in Figure 3 (i.e., D0-4, Hib-3 and AMA1-3). The mixtures were applied to AMA1(3D7) or AMA1(FVO)-coated ELISA plates as primary antibodies and the amount of antibodies which bound to AMA1 protein was measured. The mixture with 14 mg/ml of non-AMA1(3D7) IgG in this ELISA using AMA1(3D7)-coated plates had the same ratio of US-total IgG and non-AMA1(3D7) IgG as tested in Figure 3 by GIA using 3D7 strain of parasites, and 11 mg/ml of non-AMA1(FVO) IgG for FVO. As shown in Figure 5 , none of the non-AMA1 IgGs tested blocked binding of anti-AMA1 antibody in US-total IgG to the ELISA plates. The same assay was conducted using total IgG from another U.S. vaccinee and also showed no competition (data not shown). Thus it does not appear that the interfering effect is due to direct inhibition of the anti-AMA1 antibodies binding to the plate antigen.