ABSTRACT Apical membrane antigen 1 (AMA1) and the 42-kDa merozoite surface protein 1 (MSP1 ₄₂ ) are leading malaria vaccine candidates. Several preclinical and clinical trials have been conducted, and an in vitro parasite growth inhibition assay has been used to evaluate the biological activities of the resulting antibodies. In a U.S. phase 1 trial with AMA1-C1/Alhydrogel plus CPG 7909, the vaccination elicited anti-AMA1 immunoglobulin G (IgG) which showed up to 96% inhibition. However, antibodies induced by MSP1 ₄₂ -C1/Alhydrogel plus CPG 7909 vaccine showed less than 32% inhibition in vitro. To determine whether anti-MSP1 ₄₂ IgG had less growth-inhibitory activity than anti-AMA1 IgG in vitro, the amounts of IgG that produced 50% inhibition of parasite growth (Ab ₅₀ ) were compared for rabbit and human antibodies. The Ab ₅₀ s of rabbit and human anti-MSP1 ₄₂ IgGs were significantly higher (0.21 and 0.62 mg/ml, respectively) than those of anti-AMA1 IgGs (0.07 and 0.10 mg/ml, respectively) against 3D7 parasites. Ab ₅₀ data against FVO parasites also demonstrated significant differences. We further investigated the Ab ₅₀ s of mouse and monkey anti-AMA1 IgGs and showed that there were significant differences between the species (mouse, 0.28 mg/ml, and monkey, 0.14 mg/ml, against 3D7 parasites). Although it is unknown whether growth-inhibitory activity in vitro reflects protective immunity in vivo, this study showed that the Ab ₅₀ varies with both antigen and species. Our data provide a benchmark for antibody levels for future AMA1- or MSP1 ₄₂ -based vaccine development efforts in preclinical and clinical trials.